ECHINODERMATA. 23 



Family 1 — Cidauid.e. 



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

 Mouth opening wide, central ; peristome circular or pentagonal, but 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 ; the disc composed of five large equal-sized 

 angular genital plates, and five ocular plates ; anus opens in the centre of the disc, directly 

 opposite to 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, 

 which have only minute tubercles, or rows of small granules on their surface, but never 

 supporting tubercles carrying primary spines. 



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

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

 tubercle, raised on a prominent boss, and encircled by an areola, which has either a 

 round or oval form ; the areola is surrounded in general by an elevated margin, on which 

 are placed a circle of granules, the scrobicular circle, usually larger than those filling the 

 miliary zone. 



The poriferous zones are narrow, and extend without interruption from the margin of 

 the buccal membrane to the apical disc ; the pores in general are strictly unigeminal, but 

 in one genus they are bigeminal ; the pores are contiguous, or separated by septa more 

 or less thick. 



The jaws, five in number, form a very powerful lantern, which is 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, 

 however, than in the Echinidce. 



The spines in this family are large, strong, cylindrical, fusiform, prismatic, club-shaped, 

 or flattened ; their surface is sometimes covered with longitudinal lines, or with prickles 

 or granules, which in general have a linear arrangement, or they are more or less irregu- 

 larly disposed ; but the form and sculpture of the spine has invariably, as far as is known, 

 a specific value, its dominant characters being always persistent.* 



* 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 

 therewith 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, as Mill appear in the sequel. 



