THE ECHINOIDEA 307 



SUB-ORDER 2. ARBACINA. 



Diademoida iu which the ambulacra! plates are simple primaries 

 near the apical area ; at the ambitus they are compound. Some or 

 all of the compound plates consist of a large central primary, on either 

 side of which is a small demi-plate. (These plates are on the " arbacioid " 

 type of Duncan.) 



The Echinoids of this sub-order contain forms characterised by remark- 

 able simplicity of structure. The interauibulacral plates are large and 

 generally of the Cidarid type. The peristome is large. The ambulacra 

 are narrow except on the ambitus and near the peristome, where they 

 often expand somewhat suddenly. The apical system is large and simple. 

 There are two families, one of which is typically Jurassic, and the 

 other typically Cainozoic. This difference in age has probably delayed 

 the recognition of the resemblances between the two families. There 

 are, however, several Cretaceous genera which link the Jurassic and the 

 Cainozoic forms, and thus support the idea that the Arbaciidae are the 

 descendants of the Hemicidaridae. 



FAMILY 1. HEMICIDARIDAE. Arbacina in which the ambulacral 

 plates are narrow, and consist of simple primary plates for some 

 distance from the apex. _The compound plates are few in number, 

 and irregular in arrangement ; the arbacioid 

 type of plate is not extensively developed, 

 many of the compound plates being dia- 

 demoid, though with the sutures approach- 

 ing the arbacioid character (Fig. XX.). This 

 family is not well marked off from the 

 Arbaciidae. It represents the characters of 

 that family imperfectly developed. Genera " 

 Hemicidaris, L. Ag. (Fig. XVI. 4 ; Fig. 

 XX.), is the most important ; Acrocidaris, 



L \cr GnHi'maimi* T Acr rjrrfmplti? Two col P untl ambulacral 



Li. Ag. , bomopygus, U Ag. , OtrcopeWtS, p]ates of Hemicidaris intermedia, 



Pomel ; Cidaropais, Cott. ; Glypticus, L. Ag. ; Flein. ; Jurassic. Showing the 

 T , -j A. iii it. arbacioid structure of the plates. 



Leptocidans, Quenstedt, and several sub- (After Duncan.) 



genera. 



FAMILY 2. ARBACIIDAE. Arbacina with small, generally sub-conical 

 tests,- They are ornamented by numerous granules ; a bare space occurs 

 in the middle of the upper part of each interambulacrum. The am- 

 bulacral plates are mainly on the arbacioid type, but there are some 

 primaries near the apical system, and a few diademoid plates between 

 the primaries and the arbacioid plates. Ocular pores double. This 

 family includes four primitive genera ; of these two are only known 

 living, one occurs in the North American Cainozoic, and the fourth 

 ranges from the Eocene to the present day. The main distinctions 

 from the Hemicidaridae are that in the ambulacra there are fewer 

 primaries and more compound plates, and that the union of the inter- 

 arnbulacral plates is strengthened by the development of a series of 

 knobs and sockets. These occur on the facets of the plates, the knobs 

 of one plate fitting into the sockets of the next (Fig. XXI. 2). 



