PLATYBRISSUS ROEMERI. 5G3 



Platybrissus Roemeri 



! Platybrissus Roemeri Grube, I860, Jahresb. d. Sehles. Ges. f. Vat. Cult., p. CI. 



Pl.XXP.f. 3-4. 



Test depressed ; edge rounded ; outline from above regularly elliptical ; 

 vertex posterior, immediately in front of the anal system ; the test sloping 

 very slightly from the anterior extremity to the vertex. Apical system 

 anterior ; four large genital openings. Odd anterior ambulacrum scarcely 

 apparent, reduced to distant minute pairs of pores. Lateral ambulacral 

 petals open at the extremity, elongate ; poriferous zones narrow, of uniform 

 width ; pores close together; outer row largest, not conjugate. All the petals 

 are flush with the test. The anterior zones of the anterior lateral ambulacra 

 appear shorter than the posterior zones, owing to the small size of the three 

 pairs of pores adjoining the apical system. The whole abactinal part of the 

 test is covered by large secondary and miliary tubercles, distributed in the 

 same manner over all the plates : from three to five larger tubercles, sur- 

 rounded by distant miliaries, with about the same number of smaller second- 

 ary tubercles nearer the lower edge of the plates. The secondary tubercles 

 become larger towards the ambitus; and on the actinal side, which is flat, we 

 find only large, distant, primary tubercles, with few miliaries scattered be- 

 tween them. The actinostome is a segment of a circle, with the posterior 

 lip so slightly lobed that it appears on the same level with the anterior lip. 

 On the actinal side the anterior petals are only seen from the large buccal 

 tentacles radiating from the mouth in the ambulacra, the posterior lateral 

 ambulacra form a single broad avenue, occupying the whole central part of 

 the posterior part of the test ; they become separated near the posterior 

 extremity by a small pointed shield of tubercles formed by the posterior 

 interambulacral space ; but this shield is not separated either as an actinal 

 plastron or as a subanal plastron, neither of which exist in this genus. This 

 triangular shield has a slightly pointed keel, somewhat anterior to the sepa- 

 ration of the ambulacral fields on the posterior extremity. The posterior 

 extremity is bevelled anteriorly from the top of the anal system towards 

 the actinal keel. The anal system is pointed, elliptical ; neither spines nor 

 buccal or anal membranes were preserved in the specimens examined. 



Locality not known. 



