GENEBA AND QBOUPS OF THE ECHINOIDEA. 199 



Anterior ambulacra straight, the antero-lateral wide apart and 

 slightly curved. Actinallj the ambulacra are in slight hollows 

 of the test. 



Interradia large, plates numerous, bent, low and broad. 



Peristome in a slight concavity, oval, oblique, from right to left ; 

 without a floscelle or brancliial incisions. Periproct inframar- 

 ginal, elongate antero-posteriorly, ovoid or elliptical, in a groove, 

 which is more or less continued abactinally. This groove seems 

 occupied by small, elongate, distinct plates ? 



Fossil. Tithonian: Africa (Algiers). 



The author considers that the genus is near T achy cly pens, 

 from which it differs in the position of the periproct and in the 

 remarkable construction of the posterior groove. The obliquity 

 of the peristome is in the opposite direction to that oiAmhlypygus. 



Grenus G-easia, Michelin, 1854, Bev. et Mag. Zool. ser. 2, p. 439. 

 Gotteau, 1867-74, Pa?. Fran^., Terr. Jura, Ech. vol. ix. 

 p. 34. 



Test large, twice as long as broad, oval, depressed, tumid 

 dorsally and actinally, truncated posteriorly. 



Apical system very dislocated and elongate, the basal plates 

 separated by the union of the antero-lateral radial plates, the 

 postero-lateral radial plates placed far back. 



Antero-lateral ambulacra far to the front, and the postero- 

 lateral far behind, placed above the periproct ; apetaloid, flush, 

 the anterior straight and the others flexuous. 



Peristome central, circular, plain. Periproct in a deep poste- 

 rior groove which reaches the dorsum. 



Fossil. Oolite : Europe. 



There are some genera which link the Cassiduloidea and the 

 Spatangoidea in a very interesting manner, and the character of 

 the dissimilarity of the construction of the anterior and the paired 

 ambulacra is always evident, but not so much as in the true 

 Spatangoids. 



IV. Family Plesiospatakgidjs. 



Tests variable in shape and in the presence of small floseelles ; 

 ambulacra short or long in the subpetaloid parts ; apical system 

 compact or elongate and disjunct. Odd ambulacrum with pores 

 differing in shape from those of the lateral ambulacra, or they 

 may abort. 



