THE OOLITIC ECIIINODERMATA. 415 



viscera. The mouth is always below and central ; two or four rows of tubular retractile 

 suckers occupy the centre of the rays ; and in two families an anal vent opens at the 

 central or sub-central part of the dorsal surface. The complicated skeleton is composed 

 of numerous solid calcareous ossicula, variable as to number, size, and arrangement in the 

 diflferent genera which they serve to characterise. Their coriaceous integument is often 

 studded with pedicellariae and calcareous spines of various forms; they have a spongy 

 madreporiform body situated on the upper sm-face of the disc near the angle between two 

 rays ; and reptation is accomplished by retractile tubular ambulacral suckers. 



The Ophiuroidea have a distinct depressed discoidal body surrounded by long slender 

 rays, 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 supported on their sides ; they hav« no pedicellariae ; the mouth is basal and central, 

 surrounded by membranous tentacula, and they have no anal vent. 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 vertebra-like ossicula, having numerous plates or 

 spines disposed along the borders of the rays to assist in reptation. The form, structure, 

 arrangement, and covering 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, composed of a definite 

 number of plates, provided with five solid rays, independent of the visceral cavity, and 

 adapted for prehension ; they have a distinct mouth and vent, no retractile suckers, and 

 the ovaries open into special apertures at the base of the arms. The skeleton is extremely 

 complicated, being composed in some genera of many thousands of ossicula articulated 

 together, 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 stelhform plates, the articulating surfaces are sculptured with crenulations which 

 interlock into each other ; in many genera the stem was attached by a calcareous root to 

 the bed of the 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 form a net-like instrument of 

 considerable dimensions. 



The four orders of 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 following classification, which differs in 

 many essential particulars from that of previous authors. 



As the mouth is always basal, central, subcentral, or excentral, the excentricity being 

 invariably towards the anterior border, this aperture does not afford a character of primary 



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