16 TERMINOLOGY. 



the tubercle. Its margin is smooth, or crenulated, according as the summit of the boss is 

 smooth or crenulated. (PI. II, fig. 2 e.) In all the genera in which the tubercles are per- 

 forated, there is a corresponding pit in the socket of the spine for the attachment of a 

 round ligament, which passes from the tubercle to the spine. 



The head is that part of the spine which contains the articulating cavity, and is united 

 to the stem near the neck. (PI. I, fig. If; PI. II, fig. 2 e.) 



The milled ring surrounds the head. It is a prominent ridge, more or less deeply 

 crenulated, around which the muscular fibres that move the spine are firmly attached. 

 (PI. I, fig. 5 6.) 



The neck of the spine is the smooth space between the line of junction above the milled 

 ring and the rugose body of the spine. (PL I, fig. 1/.) It is often finely striated with 

 longitudinal lines. In long slender, tapering spines, (PI. IV, fig. 1 o,) the neck often 

 cannot be distinguished from the body, into which it passes. 



The stem, or body of the spine, is the part which exhibits the greatest variety of forms. 

 Smooth and muricated varieties are figured in Plates I, II, IV. 



The organs of mastication, forming the lantern of Aristotle, are rarely preserved in fossil 

 species. They consist of five jaws, each carrying a long tooth. (PI. IV, fig. 1 5,f) As 

 these parts form a complicated mechanism, their analyses and description will be given at 

 length in the anatomical part of the Introduction. 



In this descriptive terminology of the skeleton, we have limited our observations to the 

 Echinoidea, the iconography and description of the British Oolitic species of which forms 

 the first part of this Monograph: it is our intention to preface the description of the 

 Asteroidea, Ophiuroidea, and Crinoidea, which follow seriatim, with similar analyses 

 of the specialities of their organization. 



On the relative value of the external organs in the classification of the Echinoidea. 



The mouth is always basal, central, subcentral, or excentral, but the excentricity is 

 invariably towards the anterior border. This opening does not, therefore, afford a charac- 

 ter of primary importance, although, in connection with others, it is valuable in the definition 

 of families. The mouth is sometimes armed with jaws, but sometimes it is edentulous. 



The position of the anal opening affords a character of primary importance. In one great 

 section the anus opens toithin the centre of the apical disc, surrounded by the genital and 

 ocular plates. The relation, therefore, of the digestive organs to those of generation and 

 vision, is an important primary character for the zoologist. In another section the anal 

 opening is ivithout the apical disc, and is more or less external to, and at a greater or less 

 distance from, the genital and ocular plates. The physiological importance of the external 



