68 HEMICIDARID^. 



family 2 — HEMiciDARiDiE. 



Test thick, spheroidal, more or less depressed ; mouth opening large, central ; peris- 

 tome decagonal, and divided by notches, more or less deep, into ten unequal-sized lobes. 



Apical disc small, directly opposite the mouth, composed of five genital plates, and 

 five ocular plates ; the anterior pair of genital plates are larger than the posterior pair, 

 and the right antero-lateral, which always supports the madreporiform body, is the largest. 

 Anal opening round or oval in the centre of the genital circle. 



Ambulacral areas wider than in the Cidarid^, with semi-tubercles at their base only, 

 as in Hemicidaris, or extended throughout the area as in Acrocidaris ; these tubercles are 

 perforated, and provided with crenulated bosses like those occupying the inter-ambulacra ; 

 the areas are straight or undulated. 



Inter-ambulacral areas wide, composed of large plates, rarely more than eight in each 

 column ; the external surface of the plates supports large perforated tubercles, raised on 

 very prominent bosses ; the areolas are in general oblong, and confluent above and below ; 

 the incomplete scrobicular semicircles form two crescents on the sides, and they alone 

 form the narrow, central, miliary zone. One small group, Acropeltis, has the bosses 

 smooth, and the tubercles imperforate. 



The poriferous zones are narrow and undulated ; the pores are small, contiguous, 

 and unigeminal throughout, except near the peristome, where they are bigeminal and 

 trigeminal. 



The jaws are large and powerful, and armed with stout tricarinate teeth. The spines 

 in general are long, cylindrical, and tapering; sometimes they are claviform or stout, 

 compressed, and angular. Their surface is in general covered with fine longitudinal lines ; 

 but, as far as we know, neither prickles nor asperities are developed on the stem. 



I have grouped in this family the genera Hemicidaris, Acrocidaris, and Acropeltis, 

 which are all extinct, and found in the Oolitic, Cretaceous, and Tertiary Formations. 



