590 RHYNOBRISSUS. 



(HEMIASTER.) Rhynobrissus. 



Rhynobrissus A. Ac, 1872, Bull. M. C. Z., III. 



Tost thin ; outline from above diamond-shaped ; vertex posterior ; lateral 

 ambulacra petaloid. Odd anterior ambulacrum Hush with the test. Peri- 

 petalous fasciole existing with independent anal and subanal fascioles. 

 Anal fasciole forming a closed anal area. Tubercles remarkable for the 

 great development of the tlat, raised, scrobicular area. Spines of abactinal 

 surface short, silk-like, curved; on the actinal surface long, curved. Poste- 

 rior lateral ambulacra passing gradually into the actinal surface, without 

 forming a marked edge of the test in the posterior extremity. The edge 

 of the test is well defined in the anterior actinal extremity. 



Rhynobrissus pyramidaBs 



! Rhynobrissus pyramidalis A. Ac, 1872, Bull. M. C. Z., III. 



PL XXIIP.f. 4-6. 



Test thin; outline from above diamond-shaped; the greatest breadth is 

 across the apical system, which is slightly anterior to the centre. The vertex 

 is posterior, somewhat in front of the bevelled sharp posterior extremity, just 

 within the posterior edge of the peripetalous fasciole. Seen in profile the 



outline slopes regularly from the od^r. slightly arched to the vertex, curving 

 sharply from the vertex, as far as the anal system. The anal system is 

 placed in a shallow depression of the sharp bevelled posterior extremity; 

 this is truncated vertically from the subanal plastron, and joins in a 

 beak the extremity of the elongate actinal plastron, narrowest at its pos- 

 terior extremity, where it meets the diamond-shaped subanal plastron, bound 

 by a broad fasciole. The central line of the actinal plastron forms an an- 

 gular keel, which becomes more prominent towards the posterior extremity 

 of the plastron. The anterior part of the actinal surface is flat, forming a 

 sharp angle with the abactinal surface at the ambitus; in the posterior lateral 

 interambulacra the actinal surface slopes towards the broadest part of the 

 test ; the ambitus rising to form a sort of node, and then arching regularly 

 from this point towards the sharp bevelled posterior edge of the test ; the 

 actinal surface is thus reduced, in the posterior part of the test, to the width 

 of the plastron, the posterior end of which is gradually rounded, and, with- 

 out forming a distinct ambitus, passes to the vertical face of the posterior 

 half of the lateral interambulacral spaces. 



