14 PALEOPXEUSTES CRISTATUS. 



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

 some 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 tuberculation 

 of its posterior extremity passing into the tviberculation 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 flush with 

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

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

 very slightly sunken ; poriferous zones very slightl}' 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 jiart 

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

 become somewhat curved towards the ambitus, and on the actinal surfiice 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 ambulacral system of that genus as figured by Co<^ 

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

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

 figures of D'Orbigny,t the structure of the actinal surface is very different in 

 Asterostoma ; the presence of actinal grooves is so imlike 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 structui'e of Asterostoma. The figures given by D'Orbigny 

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

 Cotteau says distinctly the mouth of Asterostoma is labiate, while D'Orbigny's 

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

 tinostome. 



Faleopneustes cristatus 



! Paleopueustes cristatus A. Ac, 1873, Bull. M. C. Z., III. No. 8, p. 188. 



PI. IV. f. 1-3. 



Outline of test, seen in profile, regularly arched anteriorly ; ajiical portion 



* Desor, Synopsis Echin. foss., PL XXXIX. f. 10. 



t Cotteau, Note sur Ic genre Asterostoma Mum. Soc. Giiol. de Fr., No. II. 2^ Slt. IX. /'/. /, //. 



X D'Orbigny, Pal. f. TeiT. Crut., PL 906, 907, 908. 



