166 PROCEEDINGS OF THE MALACOLOGICAL SOCIETY. 



agree with those of Cyprimeria, as do also the other features of the 

 shell, which, however, is very small, its length from side to side being 

 only 9Jmm., and its height 9^, so that it is nearly orbicular. The 

 only other point of difference is in the posterior tooth of the right 

 valve, which, though bifid, is narrow, while in Cyprimeria it is more 

 broadly bifurcate. It occurs in the Sables de Cuise (Ypresien) and 

 in the Calcaire Grossier (Lutetien). 



This shell was regarded by M. Cossmaun in 1886 as a species of 

 ^ Mercenaria'' (now Mercimonia, see p. 169), and as no change was made 

 in his later monograph I wrote to enquire whether he was aware of the 

 resemblance of Venus obliqtia to Cyprimeria. In reply, he admits 

 that V. obliqtia has some special characters, but he still considers 

 that it belongs to the same group as Mercimonia Bernayi and 

 M. cythereceformis, " malgre de legeres differences dans la charniere." 



M. Cossmann, however, was at the same time kind enough to send 

 me a specimen of each valve of V. olliqua, and an examination of these 

 valves only confirms the opinion I had formed from his photographic 

 figures. The differences of the hinge seem to me much more than 

 slight : thus in the left valve of Mercimonia the median tooth is entire, 

 and of nearly equal thickness fi'ora end to end, differing much from 

 the bifid triangular tooth of V. ohliqua; in the right valve of 

 Mercimonia the posterior is broader and more deeplj^ cleft, and the 

 other teeth are not directed so far forward, while in both valves there 

 is a concave extension of the hinge-plate in front of the anterior teeth. 



When it is remembered that V. ohliqua agrees with Cyprimeria in 

 being compressed and suborbicular in shape, and in having a nearly 

 entire pallial line, I think that most conchologists will agree with me 

 that it should be placed under that genus, especially if it is compared 

 with such a form as Cyprimeria discus. I only regret that M. Cossmann 

 cannot yet bring himself to share this opinion. 



13. Cycloeisma, Dall. PI. VI, Fig. 9. 



This group was first separated from Cyprimeria by Conrad in 1875, 

 who proposed to call it Cyclothyris,^ but this name having been 

 appl'opriated by McCoy for a Brachiopod in 1844, Dr. Dall altered it 

 to Cyclorisma.''' The type is C. carolinensis, Conrad. 



Up to 1875 many species of Cyclorisma had been described as 

 Cyprimeria (notably by Stoliczka), because both in general shape and 

 in the dentition they agree with that genus, but the type of Cyprimeria 

 ( C. excavata, Morton) has no pallial sinus, and it was solely on this 

 ground that Cyclorisma was separated from it. 



The size and shape of the pallial sinus in Cyclorisma vary con- 

 siderably, and we find similar variations in some other early groups of 

 Veneridae, such as Tivelina and Mercimonia ; consequently we must 

 be on our guard against attributing too much importance to the 

 sinuation of the pallial line, and to the length of the siphons which is 

 connoted by it. If, therefore, this is the sole difference between the 



1 In Kerr's Eeport for 1875, Geol. Survey of N. Carolina, vol. i, App. I, p. 8. 

 - Proc. U.S. Nat. Mus., vol. xxvi (1902), p. 357. 



