GENUS — DICHELASPIS. 117 



named the occludent and basal, appear like separate 

 valves, but these, by dissection, can be most distinctly 

 seen to be united at the rostral angle. The primordial 

 valve, formed of the usual hexagonal tissue, is elliptic, 

 elongated, and placed in the direction of the occludent 

 segment ; calcification commences at its upper point, so 

 as to form the occludent segment, and afterwards at its 

 lower point, but rectangularly outwards, to form the basal 

 segment ; in the minute space between these two points 

 of the primordial valve, there is, in four of the species, 

 no calcification ; so that the two segments are united by 

 what may be called a flexible hinge ; in D. ortltogonia the 

 two calcareous segments are absolutely continuous. The 

 occludent segment is longer than the basal segment ; it 

 either runs close along the orifice, or in the upper part 

 bends inwards ; both segments are narrow, except in 

 B. Warwickii, in which the basal segment is moderately 

 broad ; the two segments are placed at an angle, varying 

 from 45° to 90°, to each other. The capitulum generally 

 extends for a little space beneath the basal segments of 

 the scuta, where it contracts to form the peduncle. 



The Terga present singular differences in shape, and 

 are described under the head of each species ; scarcely 

 any point can be predicated of them in common, except 

 that they are flat and thin. 



The Carina is much bowed, narrow, and internally 

 either slightly concave or convex and solid ; the upper 

 end extends far up between the terga ; the lower end is 

 formed by a rectangularly inflected, imbedded, triangular 

 or oblong disc, deeply notched at the end, or as in 

 H. Lowei, of a fork, the base, however, of which is wider 

 than the rest of the carina, so as to present some traces of 

 the disc-like structure of the other two species ; or lastly, 

 as in D. ort/wgonia, it terminates in a crescent-formed 

 cup. 



Peduncle. — This is narrow, compressed, and about as 

 long, or twice as long, as the capitulum; in D.Warwickii 

 it is studded with minute beads of yellowish chitine. 



