398 REPORT — 1856. 



plicated skeleton is composed of numerous solid calcareous ossicula, variable 

 as to number, size and arrangement in the different genera which they serve 

 to characterize. Their coriaceous integument is studded with calcareous 

 spines of various forms, and they have a spongy madreporiform body on the 

 upper surface of the disc near the angle between two rays ; reptation is accom- 

 plished by the retractile tubular ambulacral suckers. 



The Ophiuroidea have a distinct depressed discoidal body provided with 

 long slender arms, in which there is no excavation for any prolongation of 

 the viscera ; they are special organs of locomotion, independent of the visceral 

 cavity, and provided with spines which are developed on their sides ; the 

 mouth is basal and central, and surrounded by membranous tentacula. The 

 skeleton is composed of a series of plates which form the disc or centrum, 

 and the long slender rays are sustained by numerous elongated vertebrate- 

 like ossicula, having numerous plates or spines disposed along the borders 

 of the rays to assist in reptation. The form, structure and arrangement 

 of the discal plates, and of the ossicles of the rays, afford good characters for 

 distinguishing the genera. 



The Crinoidea have a distinct bursiform body formed of a calyx, com- 

 posed of a definite number of plates, provided with five solid rays, which are 

 independent of the visceral cavity, and adapted for prehension ; they have a 

 distinct mouth and vent, no retractile suckers, and the ovaries open at the 

 base of the arms into special apertures. The skeleton is extremely compli- 

 cated, being composed of many thousands of ossicula closely articulated to- 

 gether, the number, form and arrangement of which are determinate in the 

 different families, the multiples of five being the numbers which in general 

 predominate ; the central plate of the calyx is supported on a long jointed 

 column composed of circular, pentagonal or stelliform plates, the articulating 

 surfaces of which are sculptured with crenulations that interlock into each 

 other ; in many genera the stem was attached by a calcareous root to the bed 

 of sea, and supported the calyx and arms upwards like a plant; in others it 

 appears to have been moveable, and was used as a point of suspension from 

 submarine bodies, the calyx and arms having had a pendent position. 



The mouth is central and prominent, and the vent opens near its side ; the 

 arms are mostly ramose and multiarticulate, and when extended they formed 

 a net-like instrument of considerable dimensions. 



The four orders of the Echinodermata thus briefly described are the only ones 

 found fossil in the oolitic rocks, and of these by far the largest number of 

 species belong to the Echinoidea ; for this order I have proposed the fol- 

 lowing classification, which differs in many essential particulars from that of 

 previous authors. 



As the mouth is always basal, central, subcentral, or excentral, the excen- 

 tricity being invariably towards the anterior border, this aperture does not 

 afford a character of primary importance, although when taken in connexion 

 with others it is valuable in the definition of families. 



The position of the anal opening affords a good primary character; in one 

 section the vent opens within the centre of the apical disc, surrounded by 

 the genital and ocular plates ; in another section the vent opens without the 

 apical disc, and is external to, and at a greater or less distance from, the genital 

 and ocular plates : these two sections may be thus defined. 



Echinoidea endocyclica. 



A. Test circular, spheroioal, more or less depressed, rarely oblong ; mouth 

 central and basal ; vent in the centre of the upper surface directly opposite 



