372 LITHOTYRA VALENTIANA. 



point out the few differences. It is just possible, though 

 not probable, that this form may prove to be merely a 

 variety or younger state of L. truncata, in which case this 

 latter name would have to be sunk. The difference, 

 though one only of degree, in the form of the terga of the 

 two species is conspicuous, and there is a slight difference 

 in the carina, and again some dissimilarity in habits. 



Description. — The valves, as just stated, generally 

 resemble those of L. truncata ; scarcely any appreciable 

 difference can be detected in the scuta ; the apex, how- 

 ever, of the inner surface seems coloured a darker purple. 

 The terga, as seen from vertically above (PI. VIII, fig. 5 b), 

 have a fold or indentation on the upper or occludent 

 margin, as large and as conspicuous as that receiving the 

 margin of the scuta : this fold, as seen on the inner 

 corium- covered surface (fig. 5 a), descends below the 

 roughened knob at the upper angle of the carinal margin, 

 which is not the case with the slight fold in the same 

 place in L. truncata; its presence seems caused by the 

 edge of the central internal crest, in the upper part of the 

 carina, being square (instead of round, as in L. truncata), 

 and thus more deeply affecting the outline of the terga, 

 between which it is inserted. The upper part of the scutal 

 margin of the terga, as seen internally (fig. 5 a), overlaps 

 the scuta in a large rectangular projection. From the 

 depth of the two opposite folds, namely, that caused by 

 the tergal edge of the scuta and that by the crest of the 

 carina, the inner face of the tergum is divided into two 

 almost equal areas. The carina has its central crest square 

 (fig. 5 c, d,) instead of being rounded as in L. truncata. 

 The inner growing or corium-covered face is nearly at right 

 angles to the longitudinal axis of the whole valve, instead 

 of being oblique to it ; it is convex or protuberant, with 

 a central raised line, and two little knobs on each side 

 of the upper part ; the two lateral margins are slightly 

 hollowed out, and the basal margin is not highly pro- 

 tuberant. The rostrum is excessively minute, barely above 

 ^th of an inch in width ; it is a little enlarged at each 



