192 ROBERT TRACY JACKSON ON ECHINI. 



protractor, retractor, and radial compass muscles were attached directly to the base of the 

 intcTumbulacral plates, as restored diagrammatically in Lovenechinus (text-fig. 221). This is 

 almost exactly the condition of young Cidaris, and is virtually like that of Arbacia (text-fig. 

 227), as far as protractor and compass muscles are concerned. Muscles are not attached to 

 ambulacral areas in any known sea-urchin unless special processes, the auricles, are there 

 existent as in Centrechinoida. 



Such being the structure of the skeleton and probable position of muscles in the Perischo- 

 cchinoida, the next step in advance is seen in the Cidaridae (text-fig. 222). As Love"n (1892) 

 showed in this group, there is a direct vertical growth of the edges of the interambulacral plates 

 on the peristomal border. These growths produce wing-like expansions to which all the perig- 

 nathic muscles are attached. This arrangement is known only in the Cidaroida. As these 

 processes are a direct outgrowth of the interambulacral plates, they may properly be called 

 apophyses. They are very distinct from the separate pieces occurring on the ambulacral 

 areas of the Centrechinoida, and these may well retain the name of auricles, previously applied 

 to both. Love'n (1892, p. 38) recognized clearly the morphological distinction of apophyses 

 and auricles, but did not give any distinguishing name to differentiate them. In Eucidaris 

 (text-fig. 222) the radial compass muscles, protractors, and retractors are attached to the two 

 apophyses. The ambulacra are quite free of muscles and the character of the apophyses freed 



TEXT-FIGS. 221-230. Base of corona with attachment of lantern muscles in representative regular Echini. 



221. Lovenechinus missouriensis (Jackson). Lower Carboniferous, Missouri. X 2.2. No perignathic girdle, muscles 

 restored schematically. 



222. Eucidaris Iribuhides (Lamarck). Bahamas. R. T. J. Coll., 742. X 3. All muscles inserted on high apophyses- 



223. Phyllacanthtis baculosa (Lamarck). Mauritius. R. T. J. Coll., 753. X 2.2. Apophyses arched over the ambu- 

 lacrum, a rare aberrant variant (compare text-fig. 229). 



224. The same. R. T. J. Coll., 900. X G.5. Vertical growths on the proximal side of ambulacral plates; further 

 dorsally these growths exist as fine separate spines extending into the body cavity (compare Plate 3, fig. 12). 



225. Centrechinus setosus (Leske). Bermuda. R. T. J. Coll., 691. X 2.3. Apophyses high, auricles high, centrally 

 confluent. In this and succeeding figures protractor and radial compass muscles are inserted on more or less developed 

 apophyses, but the retractors are inserted on auricles (compare text-figs. 211, 219, 220). 



226. Phormosoma placenta Wyville Thomson. Off Cape Sable to Cape May, 956 fathoms. R. T. J. Coll., 778. 

 X 2.2. The apophysis is single, not double, as the single primordial interambulacral plate is retained; auricles have 

 secondary sutures (s). Radial peristomal (per.) and somatic (so.) muscles extend from the auricles to the peristome and 

 corona (compare text-fig. 43, p. 80). 



227. Arbacia lixula (Linnd). Naples Station. R. T. J. Coll., 886. X 2.2. Apophysis very reduced and consists of 

 a single piece as the primordial interambulacral plate is retained. Auricles are separate slender styles. 



228. Arbacia nigra (Molina). Chili. R. T. J. Coll., 797. X 2.2. Auricles meet in an arch (p. 116). 



220. Ntnmgylocentrolus drobachiensis (O. F. Miiller). Eastport, Maine. X 2.2. Apophyses low, auricles meeting 

 in an arch (compare Plate 5, figs. 1-12). 



230. Kchinometra lucunter (Linn6). Bahamas. X 2.2. Apophyses high, auricles extravagantly developed in a high 

 spoon-sha]>ed arch. 



Lettering: a, b, left and right ambulacral columns; op., apophyses; ou., auricles; pr., protractor muscles; r, radial com- 

 pass muscles; re., retractor muscles; s, suture of auricle with ambulacral plate. 



