BIIITISH FOSSILS. 



Decade IV. Plate IV. 



HYBOCLYPUS AGARICIFORMIS. 



[Genus HYBOCLYPUS. Agassiz. (Sub- kingdom Kadiata. Class Echinodermata. 

 Order Echinoidea. Family Cassidulidse.) Body ovate or suborbicular, more or less 

 expanded and depressed. Ambulacra h®mogeneous, subsimilar, the three anterior ones 

 converging and separated from the two posterior. Posterior interambulacral segments 

 with a deep longitudinal dorsal groove,' in the upper extremity of which the vent is 

 lodged. Mouth central, inferior. Tubercles minute, perforated, and placed on crenulated 

 bosses.] 



Refekence. Hyboclypus agariciformis, Forbes, — Wright in Annals 

 and Mag. Nat. Hist. 2d series, Vol. ix. p. 99. (1851.) 



Diagnosis. H. testa suborbiculari depressa sinuosd postice subtruncatd 

 ambulacris omnibus superne approxwiatis. 



This fine species, the largest of its genus with which I am 

 acquainted, is of a suborbicular or obscurely pentagonal shape, 

 expanded, depressed above, (except when young and then it is 

 gently convex,) somewhat sinuate, rather acutely margined, and 

 strikingly mushroom-like in general aspect. Beneath, it is plain 

 and slightly sinuous. Usually its breadth slightly exceeds its 

 height, and in old specimens the vertex is often depressed. Its 

 ambulacra, and also its interambulacra, are remarkably unequal. 

 The anterior or odd ambulacrum is in its widest dorsal portion 

 rather less than one fourth of the diameter of the contiguous part of 

 the antero-lateral interambulacra. The antero-lateral ambulacra 

 are slightly broader than the odd one, and narrower than the two 

 postero-laterals. The three anterior ambulacra converge around the 

 fore portion of the deeply excavated and pit-like apical disk. Their 

 terminations just impinge upon it, whereas those of the posterior 

 ' ambulacra which are sinuous and unsymmetrical, plunge into it ; 

 they are unsymmetrical chiefly in consequence of the sudden inward 

 turn made by the uppermost portions of their avenue border on the 

 anal side. The antero-lateral interambulacral segments of the back 

 are one fourth less in width than the laterals ; and these are broader 

 at their marginal portions than the posterior interambulacral 

 [iv. iv.] 4 E 



