Nicolaus Steno, or Stensen

That question trick came out nice, so I will try it again:

Who discovered the rule which describes the angles in crystals?

Who has two parts of the human anatomy named after him?

Who was the first to give a scientific explanation of fossils?

Who grew up a Lutheran in Denmark, but on his scientific quest into Italy converted to Roman Catholicism and eventually became not only a priest but a bishop?

Well, it's a trick question. His real name was Niels Steensen, but it is rendered as Nicolaus Steno, or Stensen. He was born in Copenhagen in 1638, and became Catholic in 1667. He was ordained in 1675, and made bishop two years later. As bishop, the Catholic Encyclopedia notes, "...he lived a most selfdenying and mortified life, giving all he had to the poor." He died in 1686 at age 48.

Here, in brief, are two places where the name of Steno (Stensen) has entered the sciences:

Steno's rule: The angles between equivalent faces of crystals of the same substance, measured at the same temperature, are constant.
[Dana's Manual of Mineralogy, 18th ed., 9]

Stensen's foramina: two canals for blood vessels in the upper jawbone
Stensen's duct: also called the duct of the parotid (salivary) gland
[see Gray's Anatomy 87, 108 and 885]

Next, let's examine an excerpt showing how Steno's study of anatomy enters the perennially interesting ssubject of the nature of the human brain:

...as Vesalius admitted he could "in some degree follow the brain's functions in dissections of living animals, with sufficient probability and truth; but he found himself "unable to understand how the brain can perform its office of imagining, meditating, thinking, and remembering... or however [one] may wish to divide the powers of the Reigning Soul." Steno reached a quite similar conclusion in his famous "Discourse on the Anatomy of the Brain," published in 1669. There he deplored the disparity that separated Descartes' grandiose statements about the machinery of the brain from the anatomic evidence. Not that Steno doubted the correctness of the mechanistic explanation in biology of which Descartes was a chief originator. For Steno the brain was a machine, and he emphasized that the only way to explore its workings was identical to the manner in which the contrivance of any other machine was analyzed: "I intend," he told his audience, "to show piece by piece all its parts and consider what they can do separately and together." He knew that the completion of such a task was not within his reach. He wished, however, that the brain had been as well known as many philosophers and anatomists of his day fancied, Unfortunately, he added, there were only few who spoke of man's knowledge of his brain with sufficient caution. Far larger was the number of those to whom no difficulty, however great, gave second thoughts. "These people who make assertions so promptly," Steno warned, "will give you the history of the brain and the arrangement of its parts with the same assurance as if they had been present at the construction of that marvelous machine and if they had penetrated all the plans of its great Architect." As for himself, he was "resolved to be persuaded only by those who in looking for a solid science are unable to find satisfaction in all that had been written about the brain."
His caution stood him in good stead. It earned him three centuries later the praise of another great Danish man of science, Niels Bohr, who commended his forebear's open-mindedness in recognizing the great inadequacies in man's knowledge of his brain. Happily for science, Steno's open-mindedness is still alive in many leaders of science and causes them to reach conclusions hardly different from his.
[Jaki, Brain, Mind, and Computers, 120-1; quotes are from Vesalius on the Human Brain tr. Charles Singer; "Discours sur l'anatomie du cerveau," in Vilhelm Maar (ed.), Nicolai Stenonis Opera philosophica, II; Bohr's Steno Lecture to the Danish Medical Society, Copenhagen, February 1949; published in 1957 under the title "Physical Science and the Problem of Life," in Atomic Physics and Human Knowledge]

And from another source, some more details about him:

Students came, moreover, from even the distant North of Europe to the Italian schools of medicine during these centuries. Neil Stensen, or as he is perhaps better known by his Latin name, Nicholas Steno, the discoverer of the duct of the parotid gland, which has been named after him, and of many other anatomical details, especially of the fact that the heart is a muscle, which stamp him as an original investigator of the highest order, after having made extensive studies in the Netherlands and in France to complete the medical education which he had begun in his native city of Copenhagen, went down into Italy to secure freer opportunities for original research than he could obtain anywhere else in Europe.
It may perhaps be of interest to say that while doing investigation in anatomy and certain other sciences allied to medicine, Steno became a convert to the Catholic Church and after some years became a priest. Before his ordination, however, though after his conversion, he received the call to the chair of anatomy at Copenhagen. He accepted this and worked for several years at the Danish University, but was dissatisfied with the state of affairs around him as regards religion and went back to Italy. Eventually he was made a bishop - hence the curious picture of him in a Roman Catholic Bishop's robes in the collection of pictures of professors of anatomy at the University of Copenhagen. Not long after, at his own request, he was sent up to the Northern part of Germany in order to try to bring back to the Church as many of the Germans as might be won by his gentleness of disposition, his saintly character, his wonderful scientific knowledge, and his winning ways. He is the Father of Modern Geology as well as a great anatomist, and his little book on geology was published after he became a priest, yet did not hamper in any way his ecclesiastical preferment nor alienate him from his friends in the hierarchy. He was honored especially by the Popes. In a word, his career is the best possible disproof of any Papal or ecclesiastical opposition to science in his time.

...

When Stensen, or as he is more familiarly known by his Latin name, Steno, discovered and announced the fact that the heart is a muscle, he was looked upon with very much the same suspicion as to his sanity as Harvey, a half-century before, when the great English physiologist proclaimed the circulation of the blood, and such suspicions were rather openly expressed by those who were too conservative to accept this new teaching. The heart had been considered, not figuratively as we now speak, but seriously and very literally, as the seat of the emotions. Over and over again, all men had had the experience that in times of emotional stress the heart was disturbed. They could feel their emotions welling up from their hearts, therefore there was no doubt in their minds of the truth of the old teaching. Into the midst of this perfectly harmonious concord of scientific opinion, without a dissenting voice anywhere in the world, comes a young man not yet twenty-five, who almost sacrilegiously declares that the heart is merely a muscle and not a secreter of emotions. Fortunately for him, he was of gentler disposition than most of the other men who have had the independence of mind to make discoveries, and so no very bitter opposition was aroused against him. He was considered too harmless to be taken very seriously, but at least when the announcement first came, most of those who knew anything about medicine, or thought they did, and this is much more serious in these cases, recognized that young Stensen had somehow allowed himself to be led astray into a very foolish notion, and one that could only emanate from a mind not quite capable of realizing truth as it was; and they did not hesitate to say so.
After this Stensen found the Netherlands quite an unsympathetic place for his studies, and so moved down into Italy, where he could find more freedom of thought for research and more appreciation, and continue his original investigations with less scorn for his new discoveries. Here he continued to hit upon original ideas that were likely to make things quite uncomfortable for him, not because of religious intolerance, but because of the more or less hide-bound conservatism that always characterizes mediocre minds. Far from coming into disrespect here, however, he acquired many and very close friends. He laid the foundation of modern geology and wrote a little book that is a very wonderful anticipation of supposedly nineteenth century ideas in that science. He had come down into Italy a Protestant, having been raised in that religion in his native Denmark. He found so much of sympathy with every phase of intellectual activity among the ecclesiastics in Italy, that he not only became a convert to Catholicity, but after a time a Catholic priest. His reputation spread to Rome, and the Pope not only sent for and received this innovator in anatomy and the founder of geology very courteously, but treated him with every mark of appreciation, and this within a half a century after Galileo's condemnation. Stensen eventually went back to Northern Europe as a bishop, in the hope of being able to convert to Catholicity those among the Teutonic nations who had been led away during the religious revolt.
[Walsh, The Popes and Science 96-7, 400-401]

"... in science [Steno] was centuries ahead of his time..." See here for more.