210 CPtETACEOUS PELECYPODA 



teeth small and almost obsolete in full grown specimens. The type of this sub- 

 genus is C. Grcenlandkum, Chem., the animal of which has the foot with a serrated 

 edge near its angularly bent base. The only other species Romer admits under 

 Sempes is C Mortoni, Con., which by its usually finely radiately ribbed shell 

 distinctly indicates a transition to Lcevicard'mm. Species of the type of Serripes 

 already occur in Jurassic rocks, as indicated by C. septiferum, Buvign., (Stat, de la 

 Meuse), which is, however, rather a thick shell. Erom cretaceous rocks I may 

 mention Card. Dupinianum, d'Orb., and others, but from tertiary deposits there is 

 hardly one species which could be referred to the genus. 



6. Fray urn, Bolt., 1798. Shell sub-cordate, more or less solid, with a more or 

 less distinct ridge running from the beak to the postero-inferior termination, posterior 

 end broadly truncated, slightly gaping or closed ; hinge line anteriorly curved, 

 posteriorly straight ; surface radiately ribbed, ribs with peculiar concentric scale- 

 like tubercles, more pointed or spinous in young stages than in old ones. Type, 

 Card, unedo, Linn. As far as known, species of this type are first met with in 

 cretaceous rocks, but they are rare ; their number increases slowly up to the 

 present time. 



H. and A. Adams' Ctenocardium should not be separated from Fragum. 

 E,6mer refers the so-called Cardkim australe (and donaciformej to this genus, 

 but I am very much inclined to believe that that species should be referred to 

 the DoNACiDM ; its cardinal teeth are not those of one of the Cardiid^, but entirely 

 agree with those of the former family, and the same applies to the lateral teeth ; 

 the general form of the shell also agrees with Donax. 



7. IIemicardmm,'K.lQ\n,Vlol, f^Corc?<^«<m, Bolten, apud Romer). Shell thin, 

 higher than long, heart-shaped from front or behind, laterally with a very prominent 

 ridge running from the umbones to the inferior pointed margin, surface radiately 

 ribbed ; ligament very short ; cardinal teeth of right valve more or less distinctly 

 united ; anterior laterals small ; posterior more or less elongated, especially in some 

 of the eocene species with the posterior hinge margin rather straight, as in Hem. 

 aviculare. Lam., fLithocard'mm aviculare, apud "Woodward, Man., p. 454). In 

 this and a few other allied tertiary species the anterior laterals are almost obsolete 

 and the anterior muscvilar impression very small ; type. Card. Cardissa, Linn. 



8. Lumdicardium, Gray, 1853. Shell cordate, moderately solid, inflated, with 

 a ridge extending from the umbones to the infero-posterior margin, surface radiately 

 ribbed ; lunula below the umbones deeply excavated ; cardinal teeth in right 

 valve grown together into a single horizontally flattened tooth fitting between 

 two similarly fiattened teeth of the left valve, posterior lateral teeth close to the 

 cardinal ; anterior muscular impression elevated; type, C. reti(»mn, Linn. 



I am not aware that any fossil species belonging to this genus have as yet 

 been found, but the type of the shells appears to be very old. It is possible that 

 some of them may be discovered among the numerous species of Opis described 

 from mesozoic beds. An old representative of Hemicardhim we find apparently 

 in the next genus. 



