CIDARIDJE. • 35 



Family 1 — Cidarid^. 



Test thick, turban-shaped, more or less depressed at the oral and anal apertures. 

 Mouth-opening wide, central ; peristome circular or pentagonal, without notches ; 

 aperture closed by a buccal membrane, covered with small spines, metamorphosed into 

 imbricated scales, upon which the pores from the zones are prolonged. 



Opening for the apical disc very large ; disc composed of five large, equal-sized, 

 angular, genital plates, and five ocular plates ; vent opening in the centre, directly opposite 

 the mouth ; anal membrane clothed with small angular plates, unequal in size, and variable 

 in number. 



Ambulacral areas extremely narrow, composed of a great number of very small plates, 

 having only minute tubercles, or rows of small granules on their surface, and never 

 supporting tubercles with primary spines. 



Inter-ambulacral areas very wide, composed of large plates, rarely more than from six 

 to eight in a column ; the external surface of each plate carries a large perforated 

 tubercle, raised on a prominent boss, and encircled by a round or oval areola, having 

 an elevated margin, on which are a circle of granules, usually larger than those filling the 

 miliary zone. 



Poriferous zones narrow, extending without interruption from the margin of the buccal 

 membrane to the apical disc ; pores in general unigeminal, in one genus bigeminal ; pores 

 contiguous, or separated by septa more or less thick. 



Jaws, five in number, forming a very powerful lantern, moveably connected with, and 

 supported by, a series of calcareous processes or auricles, arising from the inner surface 

 of the test ; the teeth are more simple, and the lantern less complicated than in the 

 Ecldnidce. 



The spines in this family exhibit a great variety of forms, they are large, strong, cylin- 

 drical, fusiform, prismatic, club-shaped, or flattened; and their surface is covered with 

 fine longitudinal lines, or with prickles or granules, having in general a linear arrange- 

 ment, or a more or less irregular disposition ; the form and sculpture of the spine has a 

 specific value, as its dominant characters appear to be persistent.* 



The Cidaridis are the most ancient type of the Echinoidea. The remains of different 

 forms of this family are found in the Palaeozoic rocks, as well as in those of the Secondary 



* The form and general character of the spine should, in every case, be examined with scrupulous 

 attention, and, whenever in fossil species the spines are found attached to their test, the facts connected 

 tiierewith should be noted with the greatest accuracy. The neglect of this caution has been the cause of 

 much confusion, and led to some serious errors. 



