216 ECHINODERMATA—ECHINOZOA SUB-BRANCH III 
braces ( falces, Zwischenkieferstiicke) radiating outwards from the central axis of 
the jaws; and above each brace there is a long bifid process, the rotula or 
compass (Gabelstiick). 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 
Fic. 348. solid almost to 
A, Clypeaster reticulatus, Lovén. Recent. The dental system entire, seen from their upper 
above. ‘The rotulae are placed upon the sutures of adjoining pyramids, with an : 
epiphysis on either side. Teeth in line with the mesial sutures of half-pyramids, and part, more or 
within the ring formed by the supra-alveolar crests (after Loven). B, a, Front view 4 
ofasingle pyramid. B, b, Side view of one of the half-pyramids. ; Less BS or 
re-entering on 
the outer side, and are not always of the same size. Jaws are rarely pre- 
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 


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