352 L1TH0TRYA DORSALIS. 



even only the last-formed shelly layer, is preserved, the 

 upper ones having scaled off; in a few young specimens, 

 however, all the layers were perfect. The carina is 

 generally better preserved than the other valves, and 

 hence the upper part usually projects freely ; in one spe- 

 cimen no less than ten zones of growth were preserved in 

 the carina, whilst the other valves consisted of only three : 

 the terga generally project rather more than the scuta. 

 As each growth-layer is thick, if the scaling process had 

 not taken place, all the valves would have projected greatly. 

 The little teeth lie close together on the prominent ser- 

 rated rims, on each zone of growth. The internal surfaces 

 of the valves are roughened with small imbricated points. 

 Exteriorly the valves are covered with yellow membrane, 

 with rows, corresponding with each zone of growth, of very 

 minute, yellow, horny spines, generally having their tips 

 bent over, and so made hook-shaped. These spines are 

 less than ^th of an inch in length. 



Scuta, triangular ; internally concave, with a large de- 

 pression for the adductor muscle ; there is the usual small 

 roughened internal knob, or tooth, at the rostral angle of 

 both the right and left hand valves. Tergal margin 

 straight, overlapping about one third of the entire width 

 of the terga. 



Terga, irregularly oval, with the scutal margin straight ; 

 basal point blunt, with the two sides placed at about an 

 angle of 45° to each other; the lower part of the carinal 

 margin, immediately over the latera, (as seen internally,) 

 is slightly hollowed out. Exteriorly, towards the bottom 

 of the valve, from the overlapping of the scuta, of the 

 latera, and of the carina, only a narrow rounded ridge is 

 exposed, which runs down to the basal angle at about 

 one third of the entire width of the valve, from the scutal 

 margin. Internally the valve is slightly concave. 



The Carina slightly overlaps the terga ; internally con- 

 cave ; generally with a large upper portion freely pro- 

 jecting; inwardly curved, without any central crest or 

 ridge ; valve nearly as wide as the middle part of the 



