368 LITHOTRYA TRUNCATA. 



rostral angle, in both the right and left-hand valves, in a 

 rounded, knob-like, roughened tooth. The lower part 

 of the tergal margin is slightly inflected and roughened, 

 where it meets the corresponding lower part of the scutal 

 margin of the terga. There is a deep pit for the adductor 

 muscle. The interior surface of the valve above this pit 

 is faintly-coloured purple. The inner surfaces of both 

 scuta and terga, are roughened with little points. 



Terga, seen externally, are almost quadrilateral (owing 

 to the apex being truncated), with the free margin facing 

 the scutum, arched. Seen vertically from above, each 

 shows a deep fold, which receives the lower part of the 

 tergal margin of the scutum. In the foregoing species, a 

 prominent ridge runs down the exterior surface of the 

 terga from the apex to the basal angle, against which 

 ridge, the margin of the overlapping scuta abuts : here 

 this ridge, instead of projecting straight out, is oblique 

 or folded over, and thus forms a furrow, receiving the 

 margin of the scuta. The interior growing surface of the 

 tergum (fig. 1 b\ c), presents so irregular a figure, that 

 it can hardly be described; in area it quite equals the 

 scuta ; it is slightly concave j at the upper point of the 

 carinal margin, there is a large, rounded, protuberant, 

 roughened knob, which corresponds with a small knob on 

 each side of the inner face of the carina ; these knobs seem 

 firmly united together by membrane. The scutal margin 

 of the terga, in the upper part, forms a shoulder, largely 

 projecting over the scuta; on its lower part, there is a 

 small roughened projection. The occludent margin is 

 arched and protuberant, with a slight fold above the knob 

 on the carinal margin, just mentioned : this fold is caused 

 by the protuberance of the central internal ridge of the 

 carina, but is so small, that when the capitulum is seen 

 from vertically above, it can hardly be distinguished. 

 Tin ally, the basal half of the carinal margin, runs in the 

 same line with the basal margin of the scuta. 



Carina, moderately large ; seen externally, the surface 

 presents an elongated triangle, with the apex truncated; 



