896 H. L. Haivkins — Echinoidea ffolectypoida. 



There are twenty-one or twenty-two plates in each column of the 

 petal. Three or four of the distal plates of this region show incipient 

 Pyrinid grouping, while a trifling imperfection of alternation affects 

 both columns quite near the apex. (This last may well be but an 

 individual irregularity.) The indications of structure given in 

 Agassiz' figures are therefore confirmed (broadly speaking) ; but 

 he represents Pyrinid plating as continuous to the petals in all five 

 areas, while it certainly does not extend above the ambitus in areas 

 I and V in the specimen before me, and seems not to reach the petals 

 in II and IV. 



The two features described above contain sufficient evidence for 

 generic separation of A. recens from 0. epigonus (PI. VII, Figs. 3 and 

 4), although many others could be cited. In the latter species there 

 is a perfectly definite floscelle, with expanded (though structurally 

 simple) phyllodes and well-marked, typical interambulacral 

 bourrelets. There is no trace of ambulacral complexity above the 

 phyllodes ; all ambulacrals between these and the petals are about 

 as high as broad, and in consequence the number of podial pores 

 is very small. All five petals are of the same length, and contain 

 the same number of plates ; while the petaloid pores are large and 

 perfectly similar. The name Oligojioclia is eminently appropriate 

 for epigonus, but it would be grotesque to apply it to recens, where 

 the number of ambulacral plates (and podia) is vastly in excess of 

 expectation for so small a form. 0. epigonus is a typical " Cassi- 

 duloid ", and resembles Bhynchopygus and Echinolampas in 

 ambulacral characters, as in many others. As Agassiz remarked, 

 it is closely similar to the Cretaceous genus Caratomus. The 

 affinities of Apatopygus will be discussed in the next section, but 

 a diagnosis of the genus is now possible. 



( 

 Apatopygus gen. no v., Hawkins, 1920. 



Genotype and sole species, Nucleolites recens, M. Edwards, 1836. 

 Nucleolitoida, Nucleolitidae. Test depressed, subquadrate, with 

 greatest width behind the apex. Ambitus rounded, adapical surface 

 rising to a low, almost median summit ; adoral surface slightly 

 tumid, strongly invaginated at the peristome. Apical system 

 eccentric anteriorly, built of five small oculars and four genitals, of 

 which the madreporic j)late occupies the centre of the system, and 

 just separates the posterior oculars. Peristome deeply invaginate, 

 eccentric anteriorly, transversely elli|)tical, with thin marginal plates. 

 Periproct longitudinally elliptical, situated in a deep sulcus to which 

 it forms the apicad wall, about midw^ay between the apex and the 

 posterior margin of the test. Anus surrounded by a many-plated 

 membrane, the largest plates being posterior and external. Inter- 

 amhulacra wide, built of fairly low plates bent near the median line 

 of each column. Proximal orad interambulacrals large, single, and 

 very thin. Tubercles large for a Nucleolitoid, scrobiculate, imper- 

 forate ; arranged in quincunx on apicad and orad plates. Ambulacra 



