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689 



TERTIARY FAUNA OF FLORIDA 



has made a specialty of these mollusks, he has expressed the opinion that it 

 is undescribed. It is somewhat singular that this is the only Tertiary species 

 of Unio known from east of the Mississippi in the United States. There are 

 a number of Post-Pliocene species, including those from the New Jersey 

 clays, which were long regarded as Cretaceous. 



Superfamily PECTINACEA. 



Family PECTINIDtE. 



Genus PBCTEN Miiller. 



Pfctcn (Klein, 1753) Miiller, Prodr. Zool. Dan., p. 248, 1776 ; Da Costa, Brit. Conch., p. 



140, 1778; Bolten, Mils. Bolt., p. 165, 1798; Lamarck, Prodr. d'un Nouv. Class. 



Coq., p. 88, 1799. Type Ostrca iiuixhna Linne. 



The name Pcctcn is very ancient, and appears in the prelinnean literature 

 colloquially. Although Linne himself did not formally adopt it as a genus, 

 he has used the term casually in some of his minor papers. It was first 

 introduced into binomial literature by Miiller. An excellent discussion of the 

 characters of the group by Verrill appears in the Trans. Conn. Acad. Sci., 

 vol. X., pp. 41-57, 1897. The genus has been repeatedly subdivided and the 

 number of groups which have been named, chiefly on the shell characters 

 of recent species, is very large. As might be expected, when the fossil forms 

 are taken into consideration, the groups merge into one another by insensible 

 gradations, and so far as I have been able to examine the anatomy the same 

 is true of it also, while the minor differences of the gross anatomy do not 

 appear to be at all strictly correlated with the superficial modifications of the 

 shell. Like Conns, as demonstrated by Bergh, the Pectens seem to form a 

 natural genus with a profusion of minor modifications, which may be separated 

 for convenience into sections and subgenera, but possesses within certain 

 general limits very uniform characters. The value of the named groups will 

 differ with the personal equation of those who deal with them, but it appears 

 impossible, when the fossils are included, to draw lines of generic demarca- 

 tion which shall be clear-cut yet not in violation of nature. 



In various geological horizons, as well as in the existing fauna, certain 

 species of Pccten assume a sessile habit, involving an irregular subsequent 

 growth of the valves after attachment to other objects, as in Hinnites. These 

 species have no necessary genetic connection with one another except what 

 they gain from their relations to the Pectinidce as a group, and must be regarded 

 as purely sporadic adjustments of individual forms to a particular environment. 



