248 
Rostrum almost symmetrical, moderately convex transversely, flat longitudinally; bluntly 
pointed at the apex and irregularly rounded at the basal margin; triangular in shape. On the 
inner surface, the middle part is thicker and lined on each side by a strong ridge; it reaches 
over the half of the height of the valve and it is distinctly striated horizontally. The lateral 
parts of the rostrum are also striated at the inner side, the striae are much more superficial 
here: these lateral parts are for the reception of the alae of the lateral compartments, the 
free margin of which abuts against the longitudinal ridge at the inner side of the rostrum. 
Lateral compartment has an ala on the rostral side, making an angle with the 
valve itself, the whole being, in consequence, slightly convex transversely. Longitudinally it is 
nearly flat. Parietal portion about twice as wide as in the carino-lateral compartment and of a 
triangular shape with the basal margin straight or feebly curved, and the apex truncated. The 
two lateral margins of the alae form a rather sharp angle, the apex of which is only feebly 
truncated; the lower margin is distinctly concave and extends downwards to the basis of the com- 
partment. The growth-lines on the alae are closely-set, extend obliquely upwards and make a 
curve before reaching the free margin. On the inner surface, almost at the middle of the 
parietal portion, a prominent longitudinal ledge extends from the apex over the middle on the 
one side, and not quite to the middle (on the other side) of the height of the compartment. It 
makes an angle with a horizontal ledge, the two ledges enclosing the part which on this 
valve represents the sheath. 
Carino-lateral compartment has also an ala on the rostral side, which makes 
a very feeble angle with the valve itself. For the rest, both parts are nearly flat. The parietal 
portion, which is less than or a little over half as wide as that of the lateral compartment, 
is of an elongate-triangular shape with the apex pointed, and the basal margin almost straight. 
The ala has almost the same shape as that of the lateral compartment and is about as broad 
as the parietal portion. The free margin of the ala develops into a very characteristic rim, the 
growth-lines on this rim making an angle with those on the remain part of the ala. On 
the inner surface the part representing the sheath does not extend over the parietal portion 
of the valve. The growth-lines of this part are the continuation of those seen on the narrow 
rim along the free margin at the outer surface. The narrow continuation of the ala which goes 
down to the base of the valve is distinctly thickened. 
Carina is nearly flat in longitudinal direction, only slightly curved outwards at the 
apex; it is, however, strongly bent in transverse direction. Towards the base this curvature is 
not so strong, the surface is here rounded in consequence; but towards the apex the lateral 
halves meet in a decided angle. The alae have almost the same structure and the same disposition 
as those of the carino-lateral compartments; they extend to the base of the valve; the latter, when 
seen from the inner side, presents on each side a well-developed thickened portion or rim. On 
that inner side the part representing the sheath is well-developed and extends over the parietal 
portion and over the alae as well. It is very regularly and distinctly striated horizontally. It 
stands off, in a very marked way, from the rest of the inner surface of the compartment. 
The scutum (Pl. XXVI, fig. 2, @ and ¢; fig. 5, @ and ¢) is triangular, with prominent, 
sinuous growth-ridges and with two shallow furrows radiating from the apex of the valve. Of these the 
[20 
