154 CRETACEOUS PELECYPODA 



Only the following three genera, Sunetta, Grateloif^pia, and Cytheriopsis, constitute 

 as yet the sub-family; none of the genera are known from rocks older than 

 tertiary. 



9. Simetta, Link., 1807. Shell elongated or roundish, sub -trigonal ; hinge 

 of the left valve with three cardinal teeth, the first perpendicular, very thin, the 

 second obliquely directed posteriorly and thickest, the third thin and marginal; 

 right valve with two anterior vertical cardinals and one thin sub-marginal pos- 

 terior; lunular tooth very long, raised in the left valve and fitting into a cor- 

 responding groove of the right one; pallial impression moderate, rounded or 

 sub -angular. 



The surface of the valves is either smooth or concentrically sulcated, much 

 recalling some Cytlierece and Callistce ; they are always covered with an epi- 

 dermis. There arenottw^o, but three, cardinal teeth in each valve of nearly all the 

 known species that I have examined. The roundish forms present a passage to the 

 BOSiNiiN^r Simetta meroe, Linne, is the type of the genus. Peeve in his Mono- 

 graph describes twelve species, all from the Indian and Australiean seas; three 

 other species have to be added, described since. Eossil species are very scarce in 

 tertiary deposits, and seem here already to have been represented by somewhat 

 different forms. 



The only species, anything like a Sunetta, from cretaceous rocks are, for in- 

 stance, Lucina nasuta and postradiata, Gabb, (Pal. Calif, vol. i, pi. 2-4, figs. 58-59), 

 and a few others ; but the hinges are as yet unknown. 



10. Grateloupia, desMoul., 1828, (See Homes' Eoss. Moll, von Wien, vol. ii, 

 p. 148}. Shell elongated, posterior side somewhat attenuated and narrowly 

 rounded or sub-angular at the end, hinge with three long compressed cardinal 

 teeth and one elongated lunular tooth in each valve ; of the cardinals the posterior 

 are more or less bifid, internal areal edges below the ligament greatly rugose and 

 cross grooved, sometimes with an indication of a posterior lateral tooth at the end ; 

 pallial sinus deep. Only four upper tertiary species are known, (?. irregularis, 

 Bast., being the type of the genus. 



11. Cytheriopsis, Conrad, 1865, (Am. Journ. Conch., i, pp. 7 and 146, and 

 iii, p. 146). '' Triangular; hinge composed of two compressed or linear teeth under 

 the apex and two oblique anterior to them ; in the left valve are four diverging teeth, 

 the posterior one linear, and a lateral pyramidal compressed tooth anteriorly; 

 cartilage area rugose ; pallial line with a shallow rounded sinus." In spite of the 

 differences pointed out by Mr. Conrad one cannot but entertain doubt as to the 

 real generic distinction of the only species Cyth. Ilydcma, Con. (■=Moiilinsii^ Lea), 

 from Grateloupia, under which genus Lea described it, while Conrad put it under 

 Cytherea. Lea's figure shows, strictly speaking, only three cardinal teeth in the 

 left valve, and four if we count the posterior thickened areal margin as a fourtii 

 tooth, but in such a case Grateloupia appears to me to possess more or less 

 distinctly also four cardinal teeth ! I think a strict comparison of the two shells 

 would be again most desirable, for the differences pointed out may exist merely in 

 a different wording of the characters and not in reality. 



