260 SCALPELLUM R0STRATUM, 



up to the slight prominences on their carinal margins : a 

 ridge separates, on each side, the parietes from the tectum. 



Rostrum (fig. 7 a) unusually large, about two thirds of 

 the length of the scuta, and twice as long as the rostral 

 pair of latera; internally concave, externally carinated; 

 outline of the upper portion acutely triangular, of the 

 lower portion rounded ; umbo seated at the upper end. 



Upper Latera pentagonal, with the apex rounded. 



Rostral Latera flat, four-sided, with the basal margin 

 the longest, and the baso-carinal angle produced. 



Infra-median Latera nearly equalling in area the upper 

 latera ; not descending so low down as the rostral and 

 carinal latera ; outline of lower half semi-oval, of upper 

 half rectangular. 



Carinal Latera flat, four-sided, with the basal margin 

 the longest, and slightly protuberant ; baso-rostral angle 

 produced ; whole valve larger than the rostral latus, but 

 closely resembling it in form. 



Siib-carina minute, not above one third of the size of 

 the rostral latera, which are the smallest of the other valves ; 

 internally deeply concave ; externally solid, pyramidal, 

 standing out beyond the surface of the carina, with the 

 umbo at the apex. 



The umbones of the four pair of latera are seated a little 

 above the centre in each valve, on the summit of a raised 

 triangular portion ; this arises from the valve at first 

 growing only downwards, and when added to at the 

 upper end, the new part forms a ledge at a lower level 

 round the old part, which had already acquired some 

 thickness. 



Peduncle, short, about half the length of the capitulum ; 

 narrow ; thickly clothed with minute, longitudinally elon- 

 gated, spindle-shaped, calcareous scales or beads, which 

 project but little. 



Length of the capitulum, rather under f 3 ths of an inch. 



In a Young Specimen, with its capitulum, together with 

 the peduncle, only ^th of an inch long, the scuta, terga, 

 and carina are very large in proportion to the valves of the 



