14 PALEOPNEUSTES CRISTATUS 



does not prevent its close association with Ananchytes. We have already, in 

 sonic cretaceous genera, Stenonia,* closely allied to Ananchytes and Holaster, 



a compact abactinal system. It has neither peripetalous nor anal and sub- 

 anal fascioles. The actinal plastron is elongate, triangular; the tuberculatum 

 of its posterior extremity passing into the tuberculatum of the posterior 

 interambulacral space round the anal system. Test high, conical ; sharply 

 defined ambitus ; actinal surface flat. Actinostome transverse, eccentric 

 anteriorly : posterior lip very prominent. Anterior ambulacrum Hush with 

 the test, made up of pairs of minute pores in the central part of the plates, 

 extending to the ambitus. The scmipctaloid part of the lateral ambulacra 

 very slightly sunken; poriferous zones very slightly diverging, composed, as 

 in Asterostoma, of an inner circular pore and an outer comma-shaped pore. 

 Anal system placed above the ambitus, in the truncated posterior extremity 

 of the test. Tubercles perforate, crenulate, carrying, on the abactinal part 

 of the test, straight, short spines, finely striated, with serrate edges; they 

 become somewhat curved towards the ambitus, and on the actinal surface are 

 longer, curved, spoon-shaped at the extremity, especially those of the actinal 

 plastron This genus shows even greater affinities to Asterostoma than to 

 Ananchytes. It has the ambulacra! system of that genus as figured by Cot- 

 teau.t in his note on Asterostoma The structure of the abactinal system is 

 the same. But, as far as can be judged from Cotteau's descriptions and the 

 figures of D'Orbigny.J the structure of the actinal surface is very different in 

 Asterostoma ; the presence of actinal grooves is so unlike the structure of any 

 other Spatangoids, that I have preferred establishing a new genus for this 

 species, rather than place it in a genus with the anomalous and imperfectly 

 known actinal structure of Asterostoma. The figures given by D'Orbigny 

 of the actinal surface differ essentially from the descriptions of Cotteau. 

 Cotteau Bays distinctly the mouth of Asterostoma is labiate, ■while !>'< Irbigny's 

 excellent figures, on the contrary, show a Clypeastroid or a Cassiduloid ac- 

 tinostome. 



Paleopneustes cristatus 



! Paleopneustes cristatus A. Ao., 1873, I'.nU. M. ('. 7. . Ill \ ' B, |". I s *. 



/'/. TV. f. / .;. 

 Outline ol i iii profile, regularly arched anteriorly: apical portion 



• Duos Bynopili Echin km , PI \ V\ /\ /: /<<. 



f Cottkau, Note mir li '. .1. l'i . N.. II. -j s ■ l\. PL I. II- 



| D'Orbioht, IV ! 



