194 ROBERT TRACY JACKSON ON ECHINI. 



from muscles is shown in one area. Tlie ambulacra! plates ventrallj' and throughout the area 

 passing dorsally bear very fine delicate spines that lie near the middle of the area and extend 

 into the body cavity (Plate 3, fig. 13). Ventrally near the peristome these spines may com- 

 bine into continuous ridges, as in Phjdlacanthus (text-fig. 224). Similar spines occur in some 

 Palaeozoic Echini, as Hyattechinus (Plate 24, fig. 6); but I have not seen them in the 

 Centrechinoida (p. 61). 



The apophyses in the Cidaroida in the several species seen are typically as in text-fig. 222, 

 high, broad, and flaring over the ambulacral areas. In one area of a specimen of PhyUacanthus 

 baculosa (text-fig. 223) the apophyses meet in a suture over the ambulacral area, and Duncan 

 (1885a) shows a similar case in PhyUacanthus imperialis (Lamarck). In these aberrant variants 

 the retractor muscles come to occupy a position over the ambulacrum just as they do in Cen- 

 trechinus (text-fig. 225), but with the radical difference that in Centrechinus they are inserted 

 on auricles, not on apophyses. 



In the Centrechinoida a different condition exists in the perignathic girdle. Apophyses 

 may develop on the interambulacral plates to a considerable extent (Centrechinus, text-fig. 

 225; Echinometra, text-fig. 230), or apophyses may be much reduced (Strongylocentrotus, 

 text-fig. 229; Arbacia, text-fig. 227). On the other hand a new character exists not seen in 

 the Perischoechinoida nor the Cidaroida. This feature is auricles, which, as shown by Loven 

 (1892), are two pillars, or flattened plates arising from the base of the ambulacra, but separated" 

 from the ambulacral plates by sutures. They are not a continuation of the ambulacral plates 

 as apophyses are of the interambulacral plates. Auricles originate on either side of the ambu- 

 lacrum with an open space between; they may exist as perpendicular isolated pillars (Arbacia 

 lixula, text-fig. 227; Salenia, Plate 4, fig. 5), or they may arch over the ambulacrum, joining 

 in suture {Arbacia nigra, text-fig. 228; Strongylocentrotus, text-fig. 229). The joining is a 

 feature of development, as they are apparently always separate in the young. It is also a 

 species differential and subject to individual variation, as well as to radial variation within the 

 individual. Auricles may form high and wide arching plates (Centrechinus, text-fig. 225; 

 Echinometra, text-fig. 230). When apojihyses are developed, the auricles present a suture line 

 at their point of mutual contact (Centrechinus, text-fig. 225). The auricles in this order are 

 the points of insertion of the retractor muscles of the lantern, which ai'e thus located instead 

 of being inserted on the apophyses as in the Cidaroida, a marked ordinal distinction. The 

 protractor and radial compass muscles, however, are inserted on the apophj^ses, as in the 

 Cidaroida. 



Taking up the representative types shown, in Centrechinus (text-fig. 225) the apophyses 

 are high and auricles very high, broad, and meeting in an arch over the ambulacra. The pro- 

 tractor muscles are powerful and with the radial compass muscles are attached to the apophyses. 

 The retractors are also powerful and are inserted high up on the arch of the auricles. 



