BIVALVIA. 133 



when the valves were closed : behind this ligaraental area is a large and somewhat 

 prominent tooth, which may be called a posterior lateral tooth within the dorsal margin, 

 and there is a depression outside of it, such as would receive a corresponding elevation 

 on the edge of the right valve : the posterior side of the shell is the larger, the dorsal 

 margin of which slopes with a gentle and elegant curve from a rather prominent 

 umbo, having a rounded extremity, and a gentle convexity in the ventral margin : 

 the anterior side slopes from the umbo with a straighter line, and is also rounded. 

 The shell is very thin and fragile, and was probably, when living, a semi-transparent 

 species. The muscle marks are wholly invisible. 



CRYPTODON.* Turton t 1822. 



THYASIRA. Leach, MS., 1818, fide Lam. TELLINA (sp.). Mont., 1803. 



THYATIRA. Id. MS., 1819, fide Jeffreys. 



THIATISA. Id. MS., 1819, fide Gray. 



BEQUANIA. Id. MS., fide Brown. 



VENUS (sp.). Don., 1801. 



AXINUS. J. Sower by, 1821. 



Lovtn, 1846. 



PTYCHINA. Phil, 1836. 

 THIATYRA. G. B. Sow., Jr., 1842. 



Generic Character. Shell equivalved, subequilateral, tumid, thin, subhyaline, and 

 closed : hinge with a single obtuse, or somewhat obscure tooth in each valve ; 

 no lateral teeth. Ligament semi-internal, placed in a linear depression beneath the 

 dorsal margin : impressions of the adductor muscles indistinct, that by the mantle 

 without a sinus. 



Animal with an open mantle, but no prolonged siphons, foot long, subcylindrical, 

 and tubular, with a clavate extremity. 



The Authors of the ' Hist, of Brit. Moll.' have included this in the Genus Lucina, 

 but the animal of the one species that has been examined, appears to present 

 differences sufficient to entitle it to Generic distinction, and the characters of the 

 shell are so decidedly dissimilar, that a separation seems to be required. 



There are several claimants for the honour of distinguishing this as a Genus, 

 and the strict right of priority is somewhat difficult to determine. Leach, in his 

 indiscriminate establishment of Genera, proposed several names, some of which were 

 put in print, and he may perhaps be the one most justly entitled. The name by 

 Sowerby has scarcely a better claim, as merely the exterior of the shell has been 

 observed, and the position of the ligament not clearly indicated, as Turton is next in 

 chronological order, his name is here employed. 



Species strictly belonging to this genus have not been described from any 

 Formation of an older date than the Tertiaries, the shell called Axinus obscurus, from 

 the Magnesian Limestone, belongs to a different group, and has already been made 

 a genus of by Professor King, under the name of Schizodus. 



* Etym. KpvTT-w, hidden, obovt, a tooth. 



