304 



Family 7— COLLYRITIDiE, Wrlgld, 1856. 



Famille 7me — Dysasteridees, Albin Gras, 1848. 

 Famille \re — CoLLYRiTiD^, IfOrhlgny, 1853 (pars). 

 Famille des Dysasteridees, Desor, 1856. 



The urchins forming this family have a thin test which, in general, has an ovoid, 

 elongated, or cordiform shape. The ambulacra! areas converge at two points on the 

 upper surface, which are more or less apart, whilst in all other Echinoidea, they 

 converge towards one point, the apical disc, around which are the genital apertures, 

 and the holes for the eyes. In the CollyritidtE, on the contrary, the ambulacra 

 are divisible into two groups, the anterior, composed of the single area, and the anterior 

 pair, meet at the apical disc, which is composed of the three anterior ocular, and four 

 genital plates ; the posterior, composed of the posterior pair of ambulacra, meet 

 at some distance from the former, and form an arch over the anal opening. This 

 disjunction of the ambulacra occasions a corresponding separation of the elements 

 of the apical disc, as three of the ocular plates are placed before, and two behind, at 

 the apices of the ambulacral areas ; the four genital and three anterior ocular plates are 

 intimately soldered together at the union of the three anterior ambulacral areas, and which 

 junction forms the true vertex of the test. 



The poriferous zones are narrow, the pores are unigeminal, and the tubercles in 

 general small, numerous, perforated, and crenulated. 



The mouth-opening is small, and placed near the anterior border; the peristome 

 is oval or circular, and its margin is entire ; the anal opening is round or oval, and is 

 situated in the region of the posterior border, over which the iwo posterior ambulacra form 

 an arch, the summit of which is more or less distant from the vent. 



This family forms a small, natural group of urchins, connected with the Echinocorid^ 

 on the one side, and with the Echinoconid^ and Spatangid^ on the other. Like the 

 EcHiNocoNiDiE, the ambulacral areas in the CoLLYRiTiDiE are narrow, the poriferous zones 

 complete, the pores unigeminal throughout, and the tubercles perforated and crenulated. 

 The genus Hyhoclypus connects these two families by the elongated arrangement of the 

 elements of the apical disc, and the partial disjunction of the apices of the three 

 anterior from the two posterior ambulacra in that small group. 



In the ovoidal form of the test, the structure of the ambulacra, and poriferous zones, 

 the excentrical position of the mouth, and the supra-marginal situation of the vent, 

 the CoLLYRiTiD^ resemble the genus Ilolaster, by which they are connected with 

 the family Echinocorid^, but their tM'o distinct ambulacral summits, placed at some 

 distance apart, and the consequent disjunction of the posterior ambulacra from the apical 



