TREMATOPYGUS 



Genus — Trematopygus, A. d'Orhigny, 1855. 



NucLEOLiTES (pars), Agassiz. 

 EcHixoBBissus (pars), Desor. 

 Phvllobrissus (pars), Cotteau. 

 — (pars). Be Loriol. 



Form of tlie test ovate or oblong, a little contracted and rounded before, and more or 

 less enlarged behind. 



Upper surface convex, ambitus inflated, posterior inter-ambulacrum slightly ros- 

 trated. 



Apical disc quadrate, excentral, and nearer the anterior border, composed of four 

 perforate ovarial and one single imperforate plates ; the madreporiform body covers the 

 genital elements and forms a prominence in the centre of the disc ; the five ocular plates 

 are very small and closely united to the genitals. 



The ambulacra are long, lanceolate, and well defined throughout ; the poriferous 

 zones are subpetaloidal on the upper surface ; the outer rows of the dorsal pores are 

 elongated transversely at the ambitus and base, they are equal, biserial, and microscopic, 

 and around the peristome a few supplementary pairs are present, which are larger and 

 more conspicuous than the others. 



The inter-ambulacra are built of large plates bent in the middle, having their surface 

 covered with two or three irregular rows of tubercles, which are perforated, raised on 

 mammelons, and encircled by depressed areas having circles of granules around them, 

 and the inter-tubercular surface is covered with a fine granulation. 



The anal sulcus is large, shallow, and limited to the posterior border ; the vent is 

 large, oval, or pyriform, and opens near the surface. 



The base is flat or slightly concave towards the middle ; the mouth-opening is 

 irregularly pentagonal, compressed obliquely from left to right, and from above down- 

 wards. 



Treniatoj)ijgus is only a sectional group of the genus EchinobrissKs, characterised by 

 an oblique compressed peristome, and by the large elongated vent placed in a marginal 

 shallow sulcus. 



