SCALPELLUM. 39 
together three ridges. The basal margin is bluntly pomted, with the two sides meeting 
each other at an angle of rather above 90°. Parietes rather narrow, rectangularly inflected, 
slightly concave: in the upper part there is no trace of intra-parietes. 
Terga (fig. 5, e—i). In Mr. Flower’s collection there is a tergum, (embedded in exactly 
the same matrix,) which, from a certain degree of resemblance in outline with that of 8. 
arcuatum, the nearest congener to S. frilineatum, and from another point of resemblance 
with 8. fossula, I believe belonged to this species." The valve is very smooth, with obscure 
traces of fine striz radiating from the umbo; nearly flat; pointed oval, but with the scutal 
side much more protuberant than the carial. Apex much acuminated, curled forwards ; 
carinal margin much and regularly bowed from the upper to the basal point, which latter 
is blunt and square: from it to the apex there runs, in a curved line, nearly parallel to the 
carinal margin, a barely perceptible broad ridge. Occludent margin curved up towards 
the umbo, short compared to the scutal margin ; parallel to it there runs a very wide and 
very shallow depression. Scutal margin, with a portion corresponding with the above 
depression, forming rather more than a third of the margin, not projecting so much as the 
lower two thirds, and separated from this lower part by a slight bend, probably marking 
the spot to which the apex of the upper latera extended. | 
Afinities. ‘The carma obviously most resembles that of S. fossula and arcuatum ; it 
differs plainly from both, m having a central rounded ridge: in the two well-developed 
boundary ridges of the tectum it comes nearest to the cretaceous S. fossuwla ; but in the 
absence of the intra-parietes (and this I conceive is a more important character), it comes 
nearest to the S. arcuatum, from which, however, it can be at once distinguished by the 
absence of the longitudinal strie. The tergum above described, which I believe belonged 
to this species, in the form of the scutal margin, comes nearest to that of S&. fossuda, 
though in general shape perhaps nearer to 8. arcuatum. In WS. fossula the carina has intra- 
parietes, which are closely adjusted to the straight carinal margins of the terga: in S. 
trilineatum the intra-parietes are absent, but in their place the carinal margins of the two 
terga are themselves highly protuberant, so that in these two species, although the upper 
parts of the carinz and terga are separately of very different shapes, they give, when com- 
bined together, a similar general outline. 
10. ScatprLtum simptex. Tab. I, fig. 9, 
S. carina levi; parietibus angustissimis, rectangulé infleris ; tecto subcarinato, trans- 
verse mediocriter arcuato ; margine basali rectangulé acuminato. 
’ It must however be added that the terga, at present unknown, of S. hastatum, a species occurring in 
the Grey Chalk of Dover, would probably have the same outline, and almost certainly would have a very 
smooth surface. 
