72 THE VOYAGE OF II. M.S. CHALLENGER. 



details as to their internal structure. The mule (PL VIII. fig. 8) has an elongate 

 oval shape, the surface is irregularly folded; its length was about 075 mm.; it 

 was covered by a thin chitinous mantle bearing very short hairs over its whole surface. 

 At one extremity, which no doubt corresponds to the extremity of the peduncle, a 

 couple of small knobs were seen, which probably served for the attachment to the interior 

 surface of the scutum. At the other extremity rudiments of valves were visible. I 

 could make out four of them, and they had a very regular oval shape. Between these rudi- 

 mentary valves there is probably an opening present ; a tuft of hair-like cirri protrudes 

 from this opening, and may be traced for some distance within the cavity of the mantle. 

 Of the female and hermaphrodite animal I have figured the mandible and the maxilla. 

 The mandible (PL VIII. fig. 6) has three teeth, the first of which is slightly longer than 

 the two others ; the inferior angle is a little produced and elegantly pectinated. The 

 maxilla (fig. 7) has the free edge almost straight ; two large spines are implanted near 

 the upper extremity, and another large spine is visible at a little distance from the lower 

 end. The remaining spines are small, and placed at some distance from one another. 



B. SPECIES WITH PERFECTLY CALCIFIED VALVES. 

 A. Species with a carina, a portion of which projects freely. 



Scalpettum trispinosum, n. sp. (PL VI. figs. 15, 1G). 



Valves thirteen ; surface of the valves covered by membrane. Carina simply bowed ; 

 umbo at the apex, the part of the carina where the apex is placed projects freely. 

 Rostrum and sub-carina present, sub-rostrum w T anting. Upper latus triangular, small ; 

 latera of the lower whorl two, small, triangular. 



This curious species is represented by a single adult specimen only. Two extremely 

 small specimens of the same species are inserted near its base. 



The capitulum in general shape resembles that of Scalpettum vittosum, Darwin. 

 It consists of thirteen valves — the sub-rostrum of Scalpettum vittosum is not represented 

 in Scalpettum trispinosum. All the valves are covered by a villous membrane; the 

 umbones near the apex are the only naked parts. The scuta, terga, and the carina are 

 relatively large ; the upper latera and the valves of the lower whorl are small, and placed 

 a little apart from each other. As in Scalpettum vittosum, all the valves are added to 

 at their inferior ends. 



The scutum is large, triangular. The apex projects distinctly outwards ; the 

 occludent margin is slightly hollowed out ; the basal margin is elongated. 



The tergum is large, flat, triangular, in area much surpassing the scutum. The 

 occludent margin is straight, the scutal margin convex, the carinal margin convex also. 



The carina is boat-shaped and without a flat roof. The umbo is at the apex, which 



