370 



THE ATLANTIC. 



[chap. v. 



of considerable length protrude ; what appears to be a separate 

 plate immediately behind these bears the madreporic tubercle ; 

 only two ovaries are developed, and two plates only are perfo- 

 rated for their ducts. The two posterior ambulacra end at a 

 secondary pole at a distance of about one-third of the length of 

 the shell from the primary pole near its posterior extremity. 

 The structure of the ambulacra is extremely simple, the ambu- 

 lacral canal sending a simple diverticulum to a single minute 

 pore near the centre of each ambulacral plate. The mouth is 



Fig. m.—Cahjmm relicta, W vville Thomson. Oral and apical aspects of the deuiuled test. 



unarmed. The surface of the test bears somewhat sparsely scat- 

 tered hair-like spines, and over the central portion of the oral 

 surface and on the apical surface near the posterior pole are 

 groups of delicately striated paddle-shaped spines. The general 

 color of the test and spines is pale green. Either the same spe- 

 cies, or one very nearly allied to it, was obtained in considera- 

 ble number near Tristan d'Acunha, but with a test not less than 

 200 mm. in length. The shell was, however, so extremely ten- 

 der and thin that even with the trawl not a single example was 

 got tolerably complete. 



