FROM THE UPPER GREENSAND. 257 



found in the Oolitic, Cretaceous, and Tertiary rocks, where they form important leading 

 fossils of the strata which they characterise. 



I include the following genera in this Family : 



EcHiNOLAMPAS, Gray. Echinanthus, Breynius. 



Pygurus, d'Orhigny. Conoclypus, Jyassiz. 



Faujasia, d'Orbiffny. Pygaulus, Jgassiz. 



Genus — Pygurus, tVOrbigny, 1S55. 



EcHiKANTHiTES, Leske, 1778. 



Clypeasteu (pars), Lamarck, 1801. 

 EcHiNOLAMPAS (pars), Agassiz, 1836. 



Pygurus (pars), Agassiz, 1840. 



— cVOrhigny, 1855. 



— Besor, 1858. 



— Be Loriol, 1873. 



The genus Pygurus, as now limited, is composed of large, discoidal, or clypeiform 

 Urchins, in wliich the test in general is more or less enlarged at the sides, and rostrated 

 posteriorly ; its upper surface is usually depressed, and rarely elevated. The ambulacral 

 areas and poriferous zones in the upper surface form petaloidal expansions, which have an 

 elegant figure, being in general contracted at the border, enlarged in the middle, and 

 attenuated at the apex. The anterior single area is narrower than the antero- and 

 postero-lateral areas ; the summit is in general central, or slightly cxcentral, the inclina- 

 tion being always forwards. The base is concave and much undulated, the wide basal 

 interambulacra swell into prominent cushions, and the narrow ambulacra form contracted 

 valleys between them. The mouth-opening is pentagonal, and always excentral ; the 

 peristome is surrounded by five prominent lobes, with which five expanded ambulacral 

 petals alternate ; in the poriferous zones near the mouth the pores are closely crowded in 

 triple oblique ranks ; these perforated petals form an oral rosette or a penta-phylloid 

 floscelle of considerable dimensions (PI. LVIII, fig. 1 c). 



The vent is infra-marginal ; it is in general oval, and surrounded by a distinct area, 

 which occupies the rostrated portion of the single interambulacrum ; the long diameter of 

 the opening in general corresponds with the longitudinal axis of the test, although it is 

 sometimes transverse (PI. LVIII, fig. 1 c). 



The apical disc is very small, and occupies the summit ; it is composed of two pairs of 



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