Q-ENERA AND GKOUPS OF THE ECHINOIDEA. 303 



and these may or may not be carried on as far as the true mouth ; or 

 isolated buccal plates occur. 



In the Exocyclica the peristome varies in shape and position, usually 

 actinal, and usually excentric in front ; it may be quite anterior, as in 

 Pourtalesia. It may be elliptical, decagonal, circular, oblique, pentagonal, 

 or semilunar with a posterior labrum. It has a plating upon its membrane. 



Perignathic Girdle (see Test) does not occur in Spatangoida : is continuous when 

 there are processes ambulacral in development, usually arched over, and 

 connected at their sides by ridges consisting of turned up and fused inter- 

 radial plates (Duncan, Journ. Linn. Soc. xix. p. 179) ; is discontinuous, as 

 in Gidaris and Clypeastroids, when there are either simple ridges or simple 

 processes without their union. 



Jaws. — In the Endocyclica the pyramids are five in number, and each has a 

 grooved or a keeled tooth, pointed actinally. The pyramids are in muscular 

 contact at their sides ; and at their upper junction is a " brace" radiating 

 outwards from the central axis of the jaws, which is occupied by the 

 oesophagus ; a long, bifid process, the " rotula^' or compass, is above each 

 brace. The projecting part of the pyramid bulges outwards at the 

 median line, and in some genera it is solid almost to the upper part, a very 

 small notch or foramen existing ; but in others there is a large foramen, 

 which is arched over by apophyses of the sides of the top of the pyramid ; 

 the tooth is just within this median projecting part. The jaws of the 

 Clypeastroids are difi'erently formed, and in the majority of genera overlie 

 and are rarely within the single or double discontinuous perignathic processes 

 and ridges, as the ease may be. They have each pyramid more or less 

 concave and re-entering at its median line ; the projection is at the junction 

 of two pyramids over the ambulacrum within the test, the reverse of what 

 occurs in Oidaroida for example. In Clypeastroida the jaws are low, 

 often unsymmetrical, and the teeth are aslant, or even nearly horizontal. 

 Moreover, the foramen is absent, and the compasses also, tlie braces being 

 rudimentary and not found at the upper junction of two pyramids. 



Fascicles are narrow bands of close granular ornamentation, which support rudi- 

 mentary spinules and pedicellarige. The " peripetahus" fasciole environs 

 the petaliforni parts of the ambulacra, often keeping close to them ; the 

 " lateral," or more properly termed " marginal," encircles the test about or 

 above the ambitus ; and the peripetalous fascioles may have a posterior 

 lateral branch passing beneath the anus, sometimes called ''■ infra-anal" 

 The " subanal " fasciole encloses a space or plastron perforated by pairs of 

 pores, placed beneath the anus, and it may send anal branches upwards on 

 either side of the anus, and they may enclose it, forming an anal fasciole. 

 The ^'internal fasciole" crosses the ambulacra at a greater or less distance 

 from the apical system. The internal fasciole and the subanal fasciole are 

 accompanied by modifications in the structure of the plates and tentacles 

 enclosed within their areas, and are of much importance in classification. 

 Spatangoids have fascioles, and those genera of the group without them are 



