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 

 hcre : 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 com part ment 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 alongf the free maro-in 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 (PI. XXVI, fig. 2, a and c\ fig. 5, a and c) is triangular, with prominent, 

 sinuous growth-ridges and with two shallow furrows radiating from the apex of the valve. Of these the 



