382 ECHINOZOA, 



CidaridcE. The ambulacral areas, however, though narrow and mostly 

 undulated, are wider than in the Cidaridce, and are provided with com- 

 paratively large tubercles, which may be developed inferiorly only 

 (Hemicidaris), or which may extend along the entire length of the area 

 (Acrocidaris). The interambulacral areas are wide, and carry very large 

 perforated tubercles with crenulated bosses. The spines are usually long, 

 cylindrical, and tapering. The type-genus of this family is Hemicidaris 

 (fig. 254), which ranges from the Upper Trias to the Lower Cretaceous 

 inclusive. In Acrocidaris, of the Jurassic and Cretaceous periods, the 

 ambulacral areas are comparatively wide. In the genus Pseudodiadema, 

 which ranges from the Lias to the Eocene Tertiary, the spines are solid 

 and microscopically striated, and the tubercles are perforated (fig. 253). 

 On the other hand, in the genus Cyphosoma and in a number of allied 

 forms, the tubercles are imperforate. The species of Cyphosoma are 

 mostly found in the Chalk, but some occur in the Eocene, and a single 

 species survives at present. Hemipedi7za, again, which begins in the 

 Jurassic rocks, and which is also represented at the present day by a 

 single living species, is like Pseudodiadema, but the tubercles are not 

 crenulated. This is also the case with the extensive genus Goniopygus 

 (fig. 255), which is principally Cretaceous in its range, though also occur- 



Fig. 255.— Goniopygus vtajor, viewed from above and sideways, of the natural size. 

 Cretaceous. 



ring in the Eocene rocks. In the great size of the apical disc this genus 

 reminds us of Saienia, but it wants the supernumerary " suranal " plate 

 of the latter. Among the many other genera of this sub-family, Cottaldia 

 (Chalk to Recent), Glypticus (Jurassic), Codiopsis (Cretaceous), and Tem- 

 nechintis (Pliocene) may be specially mentioned ; but the number of types 

 included in this group is too large to permit of further particularisation. 



In the sub-family of the Echinidce, the general structure of the 

 test is much the same as in the Diadematidce, but the ambulacral 

 plates consist of at least three "pore-plates" fused together, and the 

 pairs of ambulacral pores are arranged in three or more rows (rarely 

 in two rows only) on each side of each ambulacrum (fig. 256, b). 

 The test is usually globular or hemispherical, and the ambulacral 

 areas are comparatively wide, and carry two or more rows of tubercles. 

 The interambulacral areas are wide, and carry primary tubercles, 

 which are always imperforate, and are never of very large size. The 

 spines are short and awl-shaped, and their surface is marked with 

 fine longitudinal lines. 



