292 HYBOCLYPUS. 



and crenulated tubercles, and manifest a disposition to a conical elevation of the upper 

 surface of the test ; they are unlike the true type forms of this family, however, in the 

 excentral position of the mouth, the absence of notches from the peristome, in the 

 possession of a longitudinal dorsal valley, and in having the opening of the vent therein ; 

 the elongation of the apical disc, the singular disposition of its elements, and the disjunc- 

 tion of the posterior pair from the three anterior ambulacra, which converge around the 

 front of the disc, form a group of negative characters that sufficiently justify our doubts 

 as to the propriety of placing this genus in the family Echinoconid^. 



The Hyhoclypi resemble in some respects the Collyritid^, in possessing an elongated 

 apical disc, and having two ambulacral summits, consequent on the disjunction of the 

 anterior from the posterior pair of ambulacra. They have affinities with the Echinobris- 

 siD^ in having the test contracted before and expanded behind, the mouth small and 

 excentral, and the vent opening into a longitudinal valley on the dorsal surface ; but in 

 the structure of the tubercles, the apical disc, and poriferous zones, they are very diflFerent 

 from all the forms of that family. 



The Hyhodypi form an aberrant type of the Echingidea, having close affinities with the 

 EoHiNOCoNiD^ on the one side, and the Collyritid^ on the other ; they appear to form 

 a transition link between these two families, and are probably entitled to rank as a separate 

 sub-family.* Our imperfect knowledge of the intimate structure of the mouth, and the 

 presence or absence of jaws in these urchins, make it uncertain how far this separation 

 would be justified by anatomical characters, concealed or unknown. 



The diflFerent species of this genus have hitherto been only found in the Oolitic rocks ; 

 they had their greatest development in the lower division of that series, as all the English 

 species are found chiefly in the Inferior Oolite. 



A. Species from the Inferior Oolite. 

 Hyboclypus agariciformis, Tories. PI. XXI, fig. 1 a, h, c, d, e, f g. 



PrGiSTER subLjEVIS ? M'Coy, Annals and Magazine of Natural History, 2d series, 



vol. ii, p. 413. 

 Hyboclypus agariciformis. Wright, Annals and Magazine of Natural History, 2d series, 



vol. ix, p. 99. 

 — — Forbes, Memoirs of the Geological Survey, Decade V, pi. 4. 



* The genus Hyboclypus is placed by M. Desor, in his ' Synopsis des Echinides Fossiles,' p. 192, in 

 his 2d type of the family Galeridees — " Genres h appareil apicial alonge, sans plaque ginitale impaire.'" 

 I have already shown that the Hyhodypi not only possess a single imperforate ovarial plate, but that it is 

 composed of two or more pieces. I must therefore regard M. Desor's definition as a mistake, which has 

 probably arisen from the imperfect condition of the disc in the specimens he examined. 



