It's not "prima de leyes"; the expression is "catedrático de prima", and it simply meant a professor who gave his classes in the morning, specifically in the first part of the morning. I think I would keep the Spanish expression and put the explanation in English in parentheses.

"prima
14. f. Primera de las cuatro partes iguales en que dividían los romanos el día artificial, y que comprendía desde el principio de la primera hora temporal, a la salida del Sol, hasta el fin de la tercera, a media mañana."
http://lema.rae.es/drae/srv/search?key=prima

" catedrático de prima.
1. m. El que tenía este tiempo destinado para sus lecciones."
http://lema.rae.es/drae/srv/search?id=bAP4babZfDXX2GY1Lpum#c...

"Este tiempo" meaning "prima", in the sense defined: the period from daybreak to mid-morning.

It's contrasted with an "evening professor", a catedrático de vísperas:

"Que el Catedrático de Prima y Vísperas lean siempre distintas materias [...] Que el de Prima tenga leccion por la mañana de siete á ocho, y el de Vísperas por la tarde de tres á cuatro"
https://books.google.es/books?id=beApAAAAYAAJ&pg=PA47&lpg=PA...

"If Montague Cantor’s reasoning that Vivanco must have been 70 when he retired as “morning professor” [catedrático de prima] at Salamanca on January 9, 1621, is correct, then we may place his birth date at about 1551. "
https://www2.bc.edu/~noonemc/discography/manus/Noone Vivanco...

[Catedratico de Prima.—-A professor of divinity, namely, professor of prime, or morning lecturer.] [Catedratico de V esperas.-—Evening professor.]
https://books.google.es/books?id=QgQ-AQAAMAAJ&pg=RA1-PA28&lp...

As for Leyes, I would use "of Laws", which is the old academic designation in English: a number of universities in Britain and Ireland still have old-established chairs of Laws, such as the Regius Professor of Laws at Trinity College, Dublin in Ireland. I think this title would fit the period.

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Note added at 1 hr (2015-03-05 00:02:00 GMT)
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A translator's instinct is always to use a familiar term where possible, but this must not be done at the expense of failing to translate the term. A catedrático de prima was a professor who taught in the morning: there's no getting away from that. Other professors taught at other times of day. I don't think there is any equivalent to this in the universities familiar to most people, which is why it sounds odd. For that matter, most people don't know what a "catedrático de prima" was in Spanish.

Historians who write about early universities have no qualms about the term "morning professor"; on the contrary, it's perfectly familiar. See the explanations in this book, Paul F. Grendler, The Universities of the Italian Renaissance:
https://books.google.es/books?hl=es&id=UcmTz4j-XIIC&q="morni...

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Note added at 1 hr (2015-03-05 00:05:32 GMT)
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Examples from the book just cited:
"[...] the ordinary morning professor of civil law and ordinary morning professor of medical theory"
"The legists consisted of an ordinary morning professor of civil law, an ordinary afternoon professor of civil law (who normally received the highest salary), a canonist, and 3 or 4 professors who taught Institutes."
And so on.

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Note added at 1 hr (2015-03-05 00:14:33 GMT)
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And again:

"On the connotations of the term "morning professor" at Salamanca, see Caro Lynn, A College Professor of the Renaissance (Chicago: University of Chicago Press, 1937), page 89."
https://books.google.es/books?id=-moHnFxhcn0C&pg=PA339&lpg=P...