184 CEETACEOUS PELECYPODA 



the hinge has abeady been made known, I shall add here a few others which in 

 external form, at least, closely resemble some species of Trapezium and others. 



Taking all the recent species together there are not more than about 25 of 

 them known, including two or three new species from our eastern seas. Cyprhia 

 is a northern form, Glossiis and its allies may be called inhabitants of the tem- 

 perate and sub-tropical zones, while Trapezium and the forms closely allied to it 

 are mostly tropical species. The geographical range of the family is in so far 

 extensive, but the geographical distribution of the single species is not great. 



The study of this family is more important for the palaeontologist. Various 

 forms with a thin shell and more or less rudimentary hinge-teeth already occur 

 in some of the oldest sedimentary deposits ; they mostly are of the Trapezium^ 

 type. The thicker forms with better developed hinge-teeth are met with in the 

 Trias, increasing in variety in the Jurassic deposits ; in the lower cretaceous period 

 forms resembling Glossus, and especially Cyprina, are still very numerous, but in 

 the uppermost cretaceous beds the number somewhat decreases, though the same 

 types of shell still occur. In the older tertiaries the fauna has already more 

 resemblance to the present one, the number of species again somewhat decreasing ; 

 and in the newer tertiary deposits the generic forms, at least, are almost without 

 exception identical with the present ones ; the number of generic and specific types 

 being here the smallest. 



1. Modiolarca, Gray, 1840. Shell trapezoidal, moderately inflated, thin ; beaks 

 almost contiguous, incurved; anterior portion of the shell more compressed, 

 narrower and shorter than the posterior, which is obtusely carinated; surface 

 covered with a smooth shining epidermis ; hinge with one or two small, sometimes 

 almost obsolete teeth in each valve ; muscular impressions small, pallial line entire. 



H. and A. Adams (Gen., II, p. 519), propose a special family to receive this 

 genus, placing it, apparently following Gray, next to the Mytilidje; the authors 

 quote two recent species of which the type was described by Lamarck as Modiola 

 trapezina, Deshayes in his second edition of the Paris fossils (I, p. 532 and p. 541) 

 criticizes Gray's classification, and attempts to prove that there is not sufficient 

 reason for separating generically the shells in question from Trapezium ( Cypri- 

 cardiaj. Deshayes also describes two eocene species, one with two hinge-teeth, 

 and one with a single hinge-tooth in each valve. I was first inclined to class these 

 peculiar shells next to Mysia fJDiplodontaJ and the fossil JBsatliura^ but looking at 

 the animal and shells of Modlolarca, the first being characterized by partially united 

 mantle lobes, two short cirrated siphons, four gills united behind, and the other by 

 ihe peculiar trapezoid form, incurved sub-anterior beaks, small roundish muscular 

 impressions, I am inclined to follow Deshayes in his statement, that we have in 

 Modiolarca a modification of the Trapezium type of shells. At the same time 

 I cannot see how it would be possible to characterize the genus Trapezium^ if this 

 should be regarded generically the same as Modiolarca and Coralliophaga ; they 

 must all be kept distinct, and, as I said, when the fossil shells have been more 



^ Sandberger says that true Trapezia occur even in tlie Devoniau. 



