SCALPELLUxM. 23 



were probably attached. Terga (fig. 3 c) triangular, large, flat, basal angle bluntly 

 pointed; apex slightly projecting, as a solid horn ; occludent margin very slightly arched. 

 Rostrum unknown ; judging from the narrowness of the umbones of the rostral latera, 

 it was probably minute or rudimentary. Upper latera (fig. 3 e) large compared with 

 the lower valves, flat, five-sided, with the two upper sides the longest ; of the three lower 

 sides, that corresponding with the end of the rostral latera is generally (especially in 

 young specimens) the shortest. Umbo seated at the uppermost angle ; but in full-sized 

 specimens, a narrow ledge has been added, during the thickening and growth of the valve, 

 along the two upper margins, and consequently round the apex. Rostral latera (fig. 3/) 

 extremely narrow, three or four times as long as wide ; considerably arched, extending 

 parallel to the basal margin of the scuta ; widening gradually from the umbo to the 

 opposite end, which is obliquely truncated in a line (as I believe) corresponding with the 

 shortest side of the upper latera ; inner surface smoothly arched ; during growth, the narrow 

 rostral half of the valve becomes much thickened, and at the same time added to along its 

 upper margin, thus producing a solid, sloping, projecting edge ; umbo shghtly projecting. 

 Carinal latera (fig. 2>(/) almost flat, not elongated, of a shape difficult to be described; 

 approaching to a triangle, with curved sides, and one angle protuberant. 



Peduncle. The calified scales are apparently large in proportion to the valves of the 

 capitulum ; transversely elongated, pointed at both ends, and more or less crescent shaped. 



Affinities. This species was generically separated from Scalpellum by Mr. Dixon, as I 

 am informed by Mr. James Sowerby, solely owing to the umbo of growth in the carina 

 being at the apex, instead of being sub-central, as in 8. vulgare ; but I need not here 

 repeat the reasons already assigned for at present keeping all the recent and fossil species 

 under the same genus. In the umbo of growth, in the carina and scuta being seated at 

 their upper ends, in the square form of the carina, in there being only three pair of latera, 

 and in the large size of the upper latera, this eocene species is much more closely allied to 

 8. rutilum {nov. spec., of which the habitat is unfortunately not known,) than to any other 

 recent species. In some respects, however, I may remark, S. rutilum is even more closely 

 related to certain cretaceous forms. To ^S*. ornatum, the nearest recent congener of S. rutilum, 

 the present species is allied by the narrowness of the rostral latera, and by the large size 

 and peculiar shape of the scales on the peduncle : the carinal latera perhaps rather more 

 resemble those of S. vul(/are than of any other recent species. Certainly, all the affinities 

 in 8. quadratum point to 8. rutilum, ornatum, and vulgare, and these three recent species 

 are characterised by having males or complemental males attached to the sides of the orifice 

 of the sack, whereas, in the other species, they are elsewhere attached ; hence it is that I 

 believe that males were probably lodged in the slight depressions described on the inner 

 sides of the scuta ; but the depression is not here nearly so distinctly developed as it is 

 in the recent 8. ornatum, and more resembles the fold on the occludent edge of the valve 

 in ;S'. vulgare : I must add that folds of this nature do not necessarily imply the presence 

 of males. 



