GENEEA AND GEOUPS OP THE ECHINOIDEA. 203 



lacra are seen to contract above the ambitus so as to present 

 curved poriferous zones, and the small pairs commence close to 

 the margin. The postero-lateral ambulacra do not contract, and 

 are widest at the ambitus. All pores are circular. 



These are perfectly recognizable definitions, and were repro- 

 duced to a certain extent by Desor, 1858, Synopsis, p. 196, 

 pi. 37. The peristome is oval elliptical, and the small pairs of 

 pores come regularly to its margins 5 but Desor makes the radial 

 ends of the ambulacra so distant that there is room for a suspicion 

 that the basal plates were separated by the antero -lateral radial 

 plates. This is because of incorrect drawing. 



M. Cotteau noticed the genus anew in Bull. Soc. G-eol. de 

 Trance, ser. 2, vol. xxiv. p. 826 ; but the diagnosis relates to a 

 distinct type, which is now named ^seudasterostoma. The diag- 

 nosis is therefore accepted for the new genus as follows : — 



Genus Psetjdasteeostoma, gen. nov. 



Syn. Asterostoma (pars), Cotteau. 



Test large, more or less elongate, sometimes subcircular, 

 swollen above, almost flat below. 



Apical system moderately developed, subcircular ; four 

 basal and five radial plates; the antero -lateral basal plates 

 touch within ; the postero-lateral and the posterior pair of radial 

 plates are separated by a complementary imperforate plate (this 

 is the small imperforate fifth basal plate). 



Ambulacra subpetaloid, unequal ; the anterior different from 

 the others in the structure of its pores. Poriferous zones of the 

 paired ambulacra of different shaped pores, largely open abacti- 

 nally, reduced in size suddenly near the ambitus, the pairs there 

 being microscopical and very distant; near the peristome the 

 pairs become larger, closer, and more or less pronounced, 

 and converge to the mouth. Tubercles scarce, small, crenulate, 

 perforate, and scrobiculate ; granules in circles around the 

 scrobicules. Peristome sublabiate, transverse, either central or 

 very excentric in front ; periproct rounded, flush, posterior, just 

 above the margin. 



Fossil, Cretaceous ? : Cuba. 



This generic definition does not notice the definite grooving of 

 the actinal surface, the slightly contracting antero-lateral and the 

 open postero-lateral ambulacra above the ambitus, the compara- 

 tively central position of the simple peristome, and the general 



