251 



the present memoir. Their characters have been defined 

 chiefly in the works of Sars and Brady. We need ofiFer no 

 remarks except on Cytlieridea. In this genus the hinge is con- 

 structed as described by Jones (Monogr. Tertiary Entom., Pal. 

 Soc, 1856, p. 41) and Brady (Trans. Zool. Soc, vol. v., 1866, 

 p. 369), and has the following characters : — Hinge-margin of the 

 right valve marked with a series of small tubercles, which are 

 received into corresponding depressions of the opposite (left) 

 valve. These tubercles or crenulations are mostly disposed in 

 two terminal groups, and the intervening portion of the valve- 

 margin either is plain, or may be marked by minute tubercles 

 on the left, and corresponding fossae on the right valve. 

 G. S. B., I.e. 



The continuance of the crenulation along the dorsal edge 

 occurs in Ci/pideis, Jones (Monogr. Tert. Entom., p. 21); and, if 

 Worthy of being noticed as a distinctive feature, this name might 

 have been retained as subgeneric, were it not that Cytlieridea 

 perforata and C. Sorhyana (Monogr. Tert. Entom., pp. 40 and 44) 

 have it also. Moreover, Cythere lutea, Miiller, is figured by G. 

 S. Brady as having this hinge-structure in the Trans. Linn. Soc, 

 vol. xxvi., pi, xxviii., fig. 5 b, and a simple Cytheridean form of 

 hinge in Traits. Zool. Soc, vol. v., pi. Iviii., figs. 13 c,d. This shows 

 that merely by the hingement the tests of Cytheridea, Cyprideis, 

 and Cythere cannot be always clearly separated ; but, for the sake 

 of convenience in Palagontology, we must use some special terms 

 for the forms having the different kinds of hinges, as they are among 

 the few leading characters that we have at command. Thus 

 those forms having hinges with crenulated teeth and edges are 

 referred to Cytheridea, whilst those having the bar and furrow, 

 with teeth (rarely crenulated) at the angles, belong to Cythere.* 



We may also notice that the allied Cytheropteron and Loxoconcha 

 have sometimes crenulations on or near their hinges. 



* This kind of hinge is definitely described and figured in the 

 Monogr. Tert. Entom., 1856, p. 23, pi. iii. ; and in the Challenge 

 BepoH, 1880, p. 62. 



