LITHOTRYA TltUNCATA. 369 



on the internal face (fig. 1 6', d) of the free part, there is 

 (instead of being concave as is usual) a great central 

 ridge, which projects between the diverging apices of the 

 terga, as may be seen from vertically above ; hence the 

 thickness of the upper part of the carina, in a longitudinal 

 plane, almost equals its breadth. The edge of this ridge 

 is rounded. The inner or growing surface of the carina 

 is tinted purple, and lies in a plane, oblique to the longer 

 axis of the valve j it is triangular, with the apex cut off, 

 and the basal margin rounded and protuberant ; it is not 

 concave. There is a central raised line or slight ridge on 

 this inner surface, and on each side in the upper part 

 there is a small, white, roughened knob, corresponding 

 with the similar knobs on the carinal margins of the 

 terga. 



Rostrum (fig. 1 b', a), rudimentary ; in one specimen it 

 was about ^th of an inch in width ; it is either as wide, 

 or only half as wide, as the subjacent scale on the 

 peduncle. 



Latera, rudimentary, placed between the edges of the 

 carina and the terga; rather smaller than the rostrum ; 

 almost cylindrical, slightly flattened, enlarged at each 

 zone of growth, with one or two sharp teeth or spines on 

 both faces ; imperfectly calcified ; in width barely -^th 

 part of the carina. 



Peduncle, short ; the scales alone in the uppermost 

 whorl are plainly toothed; they are transversely elon- 

 gated, and almost quadrangular, and are nearly twice as 

 large as those in the second whorl. Beneath this second 

 whorl, there are two or three whorls, with scales, gradu- 

 ated in size ; and the rest of the peduncle is covered by 

 rather distantly scattered, minute, rounded or acutely 

 pointed scales : the pointed scales are directed upwards, 

 and are best developed under the carina. The basal cal- 

 careous cup, judging from two specimens, is thin, and 

 not much developed. 



Size and Colour. — The largest specimen was nearly 

 -ths of an inch across its capitulum. The calcareous 



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