334 CEETACEOUS PELECTPODA 



partially intercepting the dental series; type, _Z>. multistriata, Eorsk. Zittel 

 (Denkscli. Acad., Wien, xxiv, pt. 2, p. 165,) says that the first species of this genus 

 occurs in the Trias of the Alps. The number of species increases yery gradually 

 in the subsequent formations, but it is difficult to say whether the species are at 

 present more numerous than they were at any other time. A. Adams described 

 in Proc. Zool. Soc. for 1862, p. 229, nine recent species belonging to this genus, 

 chiefly from the China and Japan seas, and if other regions should prove to be 

 equally prolific, the genus would have its maximum of development in the 

 present time. 



4 Cyrilla, A. Ad., I860,, (olim Suxleya, A. Ad., Ann. Mag. Nat. Hist., 

 3rd ser., v, p, 303 and p. 478). Shell oblong, oblique, very inequilateral, covered 

 with a thin epidermis ; hinge with six diverging sharp teeth, directed posteriorly 

 and terminating with a curved lamina; ligament situated in a small pit under 

 the beak ; type, (7. sulcata, A. Ad., from the Straits of Korea, " dredged from 

 63 fathoms." This appears very much to recall the typical forms of the fossil 

 Trigonocoelia, (as restricted), but the hinge-teeth seem to be peculiar and few in 

 number. However, a close comparison between the two shells would be very 

 desirable. 



5. Nucinella, S- Wood, 1850, (Crag. Moll., II, p. 73, Fleurodon, Con., 

 NuGulina, D'Orbigny). Shell obliquely ovate, inequilateral, being anteriorly some- 

 what produced, closed, no perceptible hinge area ; hinge line curved with numerous 

 cross-teeth not interrupted in the middle, and an elongated anterior lateral tooth, 

 single in the right, double in the left, valve ; muscular impressions unequal, ovate, 

 anterior slightly larger; ligament very small, external, posterior to the beaks; 

 type, N, ovalis, Wood,* from miocene beds of the Vienna basin, and from the 

 crag beds of England. One species also occurs in our South Indian cretaceous 

 deposits. 



6. Axincea, Poll, 1791, fFectunculus, Lam., 1801). Shell solid, sub- orbicular, 

 sub-equilateral, or slightly oblique, more or less tumid ; hinge area distinct, 

 striated and covered with ligamental mass ; hinge line curved with numerous 

 pliciform cross-teeth, sometimes partially obsolete in the middle ; muscular scars 

 sub-equal, ovately elongated, strong, generally on somewhat raised plates ; type, 

 A, glycimeris, Linn. 



H. and A. Adams reserved Lamarck's name Fectunculus sub-generically for 

 the radiately ribbed species, but I do not think that the distinction can be upheld. 

 The fossil forms offer all stages of variation in this point of ornamentation. 



7. Lyrodesma, Con., 1841, (Ann. Geol. Report, p. 51, and Hall in Pal. of 

 New York, 1847, 1, p. 302, Actinodonta, Phil.). Shell equivalve, inequilateral, 

 anteriorly shorter and rounded, posteriorly obliquely truncate, '^ hinge with about 

 eight diverging prominent cardinal teeth, transversally striated;" ij^^, L, plana. 

 Con., from silurian rocks of North America. M'Coy (Pal. foss., &c., p. 272, pi. I k, 



* The figure of Modiola analoga ; Desliayes (Par. foss., pi. 74 figs. 27-29), would indicate that the species yather 

 belongs to Nucinella than to Crenella. It is certainly not a Modiola, though it may be a Modiolaria. 



