SGALPELLUM. 39 



together three ridges. The basal margin is bluntly pointed, 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. trilineatmn, and from another point of resemblance 

 with S.fossula, I believe belonged to this species.^ The valve is very smooth, with obscure 

 traces of fine striae radiating from the umbo ; nearly flat ; pointed oval, but with the scutal 

 side much more protuberant than the carinal. 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. 



Affinities. The carina obviously most resembles that of S. fossula and arcuatum ; it 

 differs plainly from both, in having a central rounded ridge : in the two well-developed 

 boundary ridges of the tectum it comes nearest to the cretaceous S. fossula ; 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 striae. The tergum above described, which I believe belonged 

 to this species, in the form of the scutal margin, comes nearest to that of S. fossula, 

 though in general shape perhaps nearer to S. arcuatum. In S.fossula the carina has intra- 

 parietes, which are closely adjusted to the straight carinal margins of the terga : in aS*. 

 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 carinas and terga are separately of very different shapes, they give, when com- 

 bined together, a similar general outline. 



10. SCALPELLUM SIMPLEX, Tab. I, fig. 9. 



S. carina Icevi ; parietibus angustissvmis, rectanguU inflexis ; tecto subcarinato, trans- 

 verse mediocriter arcuato ; margine basali rectanguU acuminata. 



' 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. 



