NO. 3 CASSIDULOID ECHINOIDS — KIER 89 



Location of type specimen. — Lectotype in the Museum National 

 d'Histoire Naturelle, Paris. 



Remarks. — Gauthier's figure of the adapical surface is inaccurate, 

 as it shows very flexuous petals with highly curved poriferous zones. 

 In most of the specimens the petals are straight, in a few they are only 

 slightly curved. 



Mortensen (1948, p. 189) states that "the apical system is not 

 tetrabasal, but evidently not clearly monobasal either." Evidently he 

 based this conclusion on Gauthier's figures of the apical system in 

 which the suture lines are not clear. However, in his description, 

 Gauthier states that although the sutures are not visible on all the 

 specimens, they are visible on the well-preserved specimens. On one 

 of the specimens in the Lambert Collection, I found distinct sutures. 



Genus PLAGIOCHASMA Pomel 



Plagiochasma Pomel, 1883. Class, meth., p. 59. Type species by subsequent 



designation, Melville (1952, p. i), Nudeolites olfersii L. Agassiz. 

 Synonyms : Dochmostoma Duncan ; Trematopygus d'Orbigny. 



GENERIC DESCRIPTION 



Small to medium, elongate, anterior outline smoothly rounded, 

 posterior usually slightly indented by anal groove, greatest width 

 usually posterior to center, adoral surface smoothly rounded mar- 

 ginally, depressed at peristome ; apical system tetrabasal, four genital 

 pores ; petals usually unequal with petals V and I longer than others, 

 interporiferous zones wide, poriferous zone narrow, equal length 

 in each petal, with slight tendency to close distally, all ambulacral 

 plates double pored ; periproct supramarginal, longitudinal, in groove 

 extending from opening to posterior margin ; peristome usually 

 oblique, anterior ; bourrelets absent or slightly developed ; phyllodes 

 not widened or only slightly widened, double pored, pore pairs ar- 

 ranged in two series in each half-ambulacrum; no buccal pores; 

 tubercles on adoral side only slightly larger than on adapical; no 

 naked sternal area. 



Comparison zvith other genera. — Plagiochasma is very similar to 

 Pygaulus and probably closely related. In both genera the petals are 

 open or only slightly closed, with wide interporiferous zones, narrow 

 poriferous zones, often oblique peristome, slightly developed or ab- 

 sent bourrelets, and double-pored phyllodes with the pore pairs 

 arranged in two series in each half -ambulacrum. Plagiochasma differs 

 in having a supramarginal periproct, and a less anterior apical system. 



