592 RHYNOBRISSUS PYKAMIDALIS. 



them. The actinostome is very large transversely, nearly equalling one 

 third the width of the test; it is crescent shaped ; the posterior lip not prom- 

 inent ; the bare ambulacral avenues are connected round the actinostome, 

 forming a broad, bare, anterior space, extending into a point towards the 

 edge in the lateral ambulacra, and in the posterior lateral ambulacra, and 

 as a gradually narrowing avenue on both sides of the actinal plastron; 

 the extremity of the hare posterior ambulacra widens again at its junction 

 with the subanal fasciole. at the narrowing of the posterior part of the ac- 

 tinal plastron. The anterior side of the actinal membrane is covered by 

 huge, irregular, hexagonal plates, occupying the greater part of the mem- 

 brane; the rest is covered by irregularly arranged, smaller, imbricating scales. 

 The lateral anterior ph\ lloiles are prominent ; the pores close in the odd 

 anterior one ; the phyllodes run parallel to the edge of the actinostome ; they 

 are short in the odd ambulacrum ; in the posterior ambulacra the pores are 

 distant. The broad anal fasciole extends some distance above the anal sys- 

 tem ; it commences immediately above the subanal plastron, but it is not con- 

 nected with it. The anal system is placed near the summit of the posterior ex- 

 tremity; a secondary subanal plastron is formed by a broad transverse band, 

 extending below the anal system, forming, by the extension to a point of the 

 inner part of the upper and lower part of the fasciole. two nearly discon- 

 nected, small, diamond-shaped areas. The formation of sub-areas by the 

 fascioles is thus far known only in Faorina. in which the anterior part of the 

 pcripetalous fasciole forms one. and sometimes two. independent areas. 



The anal system is elliptical (longitudinally i : the anal membrane covered 

 by numerous small plates, decreasing towards the anal opening, which pro- 

 jects, slightly trumpet fashion, beyond the level of the posterior edge. 



The color of the spines in dried specimens 18 a delicate light-gray, with 

 a rose tint. 



In a small specimen not measuring more than 17" ,m in length, and not 

 having more than eleven pairs of pores in the posterior lateral ambulacra, 

 we find no material change which is not readily explained by difference 

 in size ; the specimen fully described above measures 61 n,m . The Hat 

 scrobicular circle is not so prominently developed, and the general out- 

 line from above is less angular; in profile the anterior extremity is quite 

 abruptly truncated, and does not slope regularly from the edge as in the 

 adult. The test is extremely thin ; the ambulacral petals comparatively 

 more sunken ; the fascioles broader, as is the case in all young Spatangoids ; 



