216 



ECHINODERMATA ECHINOZOA 



SUB-BRANCH III 



braces (fakes, Zwischenkieferstilcke) radiating outwards from the central axis of 

 the jaws ; and above each brace there is a long bifid process, the rotula or 

 compass (Gabelstuck). Each pyramid has an outer concave face, and two 

 flattened sides, forming a hollow groove in which the tooth runs. The teeth 

 are correspondingly grooved or keeled, pointed actinally, and are more or 

 less vertical. 



The jaws of the exocyclic Gnathostomes are similar to those just described, 

 except in the Clypeastroids, where they are low, often unsymmetrical, and 



the teeth are 

 aslant or even 

 nearly horizon- 

 tal (Fig. 348). 

 Rotulae are ab- 

 sent, and the 

 braces are rudi- 

 mentary. The 

 pyramids are 

 solid almost to 

 their upper 

 part, more or 



A, Cl, 

 above. 



reticulatus, Loven. 

 rotnlae are placed upor 



FIG. 348. 

 Recent. The dental system entire, seen from 



the sutures of adjoining pyramids, with an 



p 



piphysis on either side. Teeth in line with the mesial sutures of half-pyramids, and 

 nthin f~ 



within the ring formed by the supra-alveolar crests (after Loven). B, a, Front view 

 of a single pyramid. B, ft, Side view of one of the half-pyramids. 



less concave, or 

 re-entering on 

 Jaws are rarely pre- 



the outer side, and are not always of the same size. 

 served in the fossil state. 



All Echinoids having a dental system are provided also with a perignathic 

 girdle (also termed auricles). This is a structure composed of ambulacral 

 processes and interambulacral ridges, which together surround and underlie 

 the jaws, and furnish attachment for their muscles. The girdle is continuous 

 when the ambulacral processes are arched over and connected at their sides 

 by ridges consisting of turned up and fused interambulacral plates ; and 

 discontinuous, as in Cidaris and the Clypeastroids, when there are either simple 

 ridges or simple processes without their union (Duncan). The girdle is 

 developed from the test itself, and does not therefore belong to the dental 

 system. 



Tubercles and Spines. The plates of Echinoids are almost always covered 

 with wart-like tubercles and granules, which carry various kinds and sizes of 

 spines. The larger and completely developed tubercles are called primaries; 

 those of a smaller size are secondaries; and very small tubercles, sometimes 

 incomplete in their development, are miliaries. Granules are irregular or 

 nodular projections of the test ; they may be large and widely separated, or 

 very numerous and of various sizes. The base of a tubercle is termed the 

 boss, and its upper part may be either plain or crenulated. The boss supports 

 a rounded mamelon, which is said to be perforated when pierced by a central 

 foramen for a slight distance, or imperforate when it is not. A plain or 

 sunken space surrounding the base of the tubercle is called the scrobicule, or 

 areole ; its outer limit, the scrobicular circle, is generally marked by a ring of 

 granules, but in many cases the scrobicules of the same meridian are con- 

 tinuous. Secondary tubercles may or may not be scrobiculate. 



All the tubercles of Sea-urchins bear movable spines, which vary greatly 

 in dimensions, and in the shape and nature of their cross-sections. They are 



