NO, 3 CASSIDULOID ECHINOIDS — KIER 9I 



National d'Histoire Naturelle, one in the ficole National Superieure 

 des Mines, Paris, and one in the U. S. National Museum. 



Shape. — Small to medium, up to 30 mm. long, elongate, smoothly 

 curved anterior outline, slightly truncated posterior, sides expanding 

 posteriorly with greatest width posterior to center ; adapically inflated 

 anteriorly, with steeply sloping anterior surface, gradually sloping 

 posterior; adorally depressed around peristome, ambulacra slightly 

 depressed, ambulacrum III more depressed than others. 



Apical system. — Very eccentric anteriorly, tetrabasal (text fig. 74), 

 genital plate 2 extending posteriorly, separating genital plates i and 4 ; 

 ocular plates very large, ocular plates I and V in contact. 



Ambulacra. — Petals broad, flush, unequal in length, petal III 

 shortest, petals V and I longest, in specimen 22 mm. long: 32 pore 

 pairs in poriferous zone of petal III, 36 in petals II or IV, 47 

 in V or I ; petals slightly closing distally, petal III more open 

 than others; interporiferous zones very wide, twice width of porifer- 

 ous zones; poriferous zones taper distally, of equal length in same 

 petal, pores conjugate, outer pores elongated transversely, inner 

 round. 



Adoral interambulacra. — Single plate (text fig. 75) at peristome, 

 preceded by regularly alternating plates. 



Periproct. — Supramarginal, longitudinal pointed anteriorly, ex- 

 panding and smoothly rounded posterior; in groove extending to 

 posterior margin. 



Peristome. — Large, depressed, anterior, oblique, with long axis 

 extending from left anterior to right posterior. 



Ploscelle. — Bourrelets slightly developed ; phyllodes (text fig. y6) 

 narrow to slightly widened, double pored with two series of pore 

 pairs in each half -ambulacrum : five to seven pore pairs in each outer 

 series, three to five in each inner ; no buccal pores. 



Tuberculation. — Adorally tubercles slightly larger than adapically. 



Occurrence. — Lower Cretaceous (Hauterivian) of France and 

 Switzerland. 



Location of type specimen. — Not known. 



Remarks. — d'Orbigny (1857, p. 374, pi. 949, fig. 7) described and 

 figured a fifth imperforate supplementary plate in Plagiochasma 

 olfersii. Wright (1875, Cretaceous, p. 252) mentioned this plate in 

 his generic description, but presumably he was basing his description 

 in part on d'Orbigny's, because he does not report this plate in his 

 description of the apical system in Plagiochasma faringdonense 

 Wright. This plate was not present on any specimens I studied of 

 P. olfersii, and Melville (1952, p. 4) did not find it on any specimens 



