530 ARACHNOIDES PLACENTA. 



Arachnoides placenta 



Echinus placenta Lix.v., 1758. Syst, Xat. 



\ Arachnoides placenta Agass., 1841, Mon. Sent., p. 94. 



PL Xlir.f. 1-4. 



Gray has attempted to distinguish two species in this genus; a careful 

 comparison between his specimen and specimens of the same size of A. pla- 

 centa show no differences which can be considered as specific. The outline 

 is, seen from above, slightly angular, nearly circular. The edge of the test is 

 thin, the profile slightly conical ; the greatest diameter immediately across the 

 extremity of the posterior pair of petals; the poriferous zones are narrow, 

 nearly of uniform breadth the whole length ; the poriferous grooves are deep ; 

 the ambulacra! regions are broader than the interambulacral, both on the 

 actinal and abactinal part of the test. The petals can scarcely be called 

 such, as the poriferous zones diverge regularly from the apex towards the 

 outer edge, curving even outwardly at the extremity towards the median 

 interambulacral space. The conjugated portion of the poriferous /ones ex- 

 tends to about half-way between the apex and the edge. The apical system 

 is compact, slightly raised, like a button, above the level of the depressed 

 interambulacral spaces, which are especially sunken near the apex between 

 the poriferous zones. The amis is supra-marginal near the edge ; in 

 younger specimens it is somewhat more distant. The peristomal star is 

 small, remarkably well defined, regularly pentagonal. The posterior genital 

 opening is frequently wanting. Apex slightly posterior; anus angular, 

 comparatively smaller than in other genera of the family ; outline slightly 

 indented opposite anus, forming projecting angles; the young are quite 

 pentagonal. 



New Zealand ; Australia ; East India Islands ; Burmah. 



