DICHELASPIS ORTHOGONIA. 131 



General Appearance. — Capitulum apparently much 

 flattened ; valves naked, coloured reddish, separated from 

 each other by thin structureless membrane. 



The Scuta consist of two bars placed at right-angles to 

 each other, with the point of junction fully as wide as 

 any part of the basal segment, and perfectly calcified ; 

 the primordial valve lies at the bottom of the occludent 

 segment. The basal segment is equally narrow through- 

 out, and very slightly concave within ; the occludent 

 segment widens a little above the junction or umbo, and 

 then keeps of the same width to the apex, which is 

 obliquely truncated j internally this segment is concave ; 

 externally it has a central ridge running along it ; the 

 occludent segment is twice as long and twice as broad as 

 the basal segment. Both segments are a little bowed 

 from their junction to their apices. 



Terga. — These are of a singular shape ; they are about 

 three-fourths as long as the occludent segment of the 

 scuta, and in their widest part, of greater width than it. 

 They consist of four prominent ridges proceeding from 

 the umbo, and united together for part only of their 

 length, and, therefore, ending in four prominences ; one 

 of these, the longest, has the same width throughout, 

 and forms the basal point ; a second, very small one, is 

 seated high up on the carinal margin just above the apex 

 of the carina ; the third and fourth, are nearly equal in 

 length, and project one above the other on the scutal 

 margin. There are two occludent margins, meeting each 

 other at right angles, and forming a prominence, as in 

 Lepas ; and this gives to the margin of the valve the five 

 prominences. The whole valve internally is flat ; exter- 

 nally, it is ridged as described. 



Carina (fig. 10, a, 6), much bowed, narrow, long; 

 externally, the central ridge is quite flattened ; internally, 

 slightly concave, but scarcely so towards the lower part, 

 which is narrow ; the upper part widens gradually, and the 

 apex is rounded. The basal embedded portion is as 

 wide as the uppermost part, and forms a cnp, unlike any- 



