GENERA AND GROUPS OP THE ECHINOIDEA. 49 



ambulacral processes. Jaws large, foramen large, without epi- 

 physes ; teeth grooved. Spines often large, variable, and usually 

 solid. 



Genus Hemicidaris. 



Subgenus Hemidiadema. 



„ Hypodiadema. 



„ Pseudocidaris. 



„ Asterocidaris, 



Genus Acrocidaris. 



Subgenus Acropeltis. 



Genus Goniopygus. 



„ Circopeltis. 



„ Cidaropsis, 



„ Glypticus. 



Genus incertse sedis : Leptocidaris, Quenst, 



Genus Hemicidaris, Agassiz, 1840, EcJi. Foss. de la Suisse, pt. ii. 

 p. 42. Desor, 1858, Synopsis, p. 50. Duncan, 1885, Quart. 

 Journ. Geol. Soc. vol. xli. p. 43G. (Amended.) 



Syn. Ttaris, Quenst. ; Semipygus, JEtallou (immature). 



Test of moderate size, spheroidal, tall, tumid at and above the 

 ambitus, flattish and often broadest actinally. 



Apical system small, either with large united basal plates (or 

 some or all radial plates enter the periproctal ring*). 



Ambulacra narrow, enlarging near the ambitus, projecting or 

 not ; the plates a multitude of small low primaries near the apical 

 system, succeeded by compound plates which may be formed by 

 two to four constituents, their arrangement partly ' ' Diadem- 

 atoid," but with additional primary or demi-plates, placed abac- 

 tinally to the large tuberculous primary plate. Pairs of pores 

 simple abactinally, in arcs near the large tubercles, and crowded 

 near the peristome. Tubercles in two vertical rows, perforate 

 and crenulate, large to varying distances above the ambitus, 

 and then diminishing in size or replaced by granules. 



Interradia broad, with but few high coronal plates, with two 

 vertical rows of large, projecting, perforate and crenulated primary 

 tubercles, larger than those of the ambulacra. Scrobicules usually 

 contiguous and incomplete ; secondary tubercles and granules. 



Peristome large and the branchial incisions well developed. 

 Pyramids of the jaws with a large incomplete foramen; teeth 



* Very rare, or probably in subgeneric groups only. 

 lilNN. JOURN. — ZOOLOGY, VOL. XXIII, 4 



