156 THE VOYAGE OF H.M.S. CHALLENGER. 



a lono-itudiual ridge from which the sides of the shell slope off with a pleasing curve runs 

 along the apical surface. The oral surface is nearly flat, and a slight keel runs round its 

 edge, defining and limiting it very much as in Ananchytes, a fasciole follows the keel, 

 only leaving it and appearing for a little part of its course on the oral surface in advance 

 of the mouth. The mouth is oval, its long axis in the direction of the antero-posterior 

 axis of the test. The excretory opening is on the posterior surface above the line of the 

 peripheral ridge. The apical area is, if not disjunct, greatly produced, but it is difiicult 

 to make out the exact relations of some of the terminal plates of the ambulacral and 

 interambulacral series. The ambulacra of the trivium meet at an anterior pole on the 

 dorsal surface nearly opposite the mouth, and the two ovarial plates closing the two 

 anterior interambulacral series bear large ovarial openings from which, as in Aerope, 

 tubes of considerable length protrude ; what appears to be a separate plate, immediately 

 behind these bears the madi-eporic tubercle, only two ovaries are developed, and two 

 plates only are perforated 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 ambulacral canal sending a simple diverticulum to a single minute pore near 

 the centre of each ambulacral plate. The mouth is unarmed. The surface of the test 

 bears somewhat sparsely scattered 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 colour of the test and spines is pale green. 

 Either the same species, or one very nearly allied to it was obtained in considerable 

 numbers near Tristan da Cunha, but with a test not less than 200 mm. in length. The 

 shell was, however, so extremely tender and thin that even with the trawl not a single 

 example was got tolerably complete." 



The actinostome is pentagonal (PI. XXXIV. figs. 7, 8), and is strengthened by 

 irregularly concentric rows of plates, the larger on the exterior edge. 



Seen in profile (PI. XXXIV. fig. 3), the course of the lateral fasciole on the edge of 

 the slight keel, which marks the ambitus as it were, the line between the actinal and 

 abactinal surfaces is well shown. This lateral fasciole is somewhat broadest as it crosses 

 the posterior extremity of the test (PI. XXXIV. fig. 4). 



The concentration of primary tubercles above the subanal beak to form an indistinct 

 subanal fasciole is very marked. These tubercles pass from primaries to secondaries, and 

 then to miliaries. It is the only case known to me of a closed area thus changed into 

 a plastron surrounded by what corresponds to a fasciole, but made up of primary tubercles. 

 The formation of an ambital fasciole in Pho7'mosonia by the concentration of secondary 

 tubercles seems to be an analogous case to this. Only two genital openings are developed 

 (PL XXXIV. fig. 2) ; the apical system is disconnected ; there are probably three interca- 

 lated interambulacral plates separating the Itivium from the trivium, but owing to the 



