376 Mr. T. H. Withers on the 
TI. 15447-I. 15456, and another more important sct com- 
prising two carinz (I. 15443-4), one scutum (I. 15445), and 
a tergum (I. 15446). 
Measurements.—The following are the measurements of 
the scutum and tergum, and the largest carina :— 
Length. Breadth. 
mm. mm, 
Carina (Pl. V. fig. 15) .... 22:1 (when complete pro- 11:2 
bably about 25 mm.). 
Scutum (Pl. V. fig. 11).... 18:2 9:3 
Tergums (PV nig 12) ia 8:4 
Description.— All the valves of this species seen by me 
from Essen are much worn, and the comparatively coarse 
sand-grains composing the matrix have in most cases pitted 
the surface of the valves. From certain valves that are 
well preserved it would appear that the surface was origin- 
ally smooth, except fer the faint transverse growth-lines. 
Carina triangular, unusually wide, about twice as high as 
wide, subcarinated, transversely semicircular, with the lateral 
margins slightly inflected, basal margin angular, formed of 
two curved lines making an angle of about 90°, the outer 
angles being produced into short spurs ; the apical part of 
the valve is almost always bowed outwards to a variable 
extent. The growth-lines follow the outline of the basal 
margin, but on the inflected lateral edges they are slightly, 
but rather abruptly, upturned. On the inner surface the 
lower two-thirds of the valve is deeply concave, and the upper 
third of the valve is thick and solid, and no doubt freely 
projected to that extent. Two ridges, much more prominent 
in some valves than in others, and thickest in the middle, 
are produced on the solid apical portion, and they project 
beyond the lateral margins, from which they are separated 
by a distinct furrow. 
Scutum subtriangular, moderately convex transversely, 
with the tergo-lateral portion abruptly bent downwards and 
inwards from the somewhat raised and rounded apico-basal 
ridge ; occludent margin moderately convex ; basal margin 
almost straight, about half the width of the occludent 
margin. On the inner surface the inner occludent edge is 
moderately wide and marked with longitudinal lines; there 
is no very deep pit for the adductcr muscle, but above the 
pit the valve is thick and solid ; the apical part of the tergo- 
lateral edge is broken in this specimen, but it appears to 
have been much produced inwards, forming a fairly deep 
furrow between it and the flattened inner occludent edge. 
Tergum subrhomboidal with a straight apico-basal ridge, 
