30 ROBERT TRACY JACKSON ON ECHINI. 



embraced and held tightly in place by one of the pyramids, and proximally the soft growing 

 end is enveloped by the dental capsule (Plate 5, figs. 1, 6). The pyramid dorsally includes a 

 triangular space, the foramen magnum, which is shallow, or deep, and dorsally open, or closed 

 by the lateral extension of the epiphyses (text-figs. 207-216, p. 184). The two half -pyramids, 

 forming one pyramid and joined by suture, are interradial in position (text-fig. 211, p. 184); 

 or the two half-pyramids joined by an interpyramidal muscle are radial in position (text-fig. 220, 

 p. 191). These radial half-pyramids are a feature connected with the view of considering a 

 sea-urchin as composed of five radial parts (pp. 62, 190, text-figs. 217-220). Each half-pyramid 

 bears on its inner face an elevated ridge, the dental slide, which supports the tooth (Plate 2, fig. 

 10); and has a lateral wing that extends to the oesophageal cavity; on the outer face of the 

 wing in regular Echini are ridges for the attachment of interpyramidal muscles (Plate 5, figs. 

 3, 5, 9). The dorsal face of a half-pyramid, seen when the epiphysis is removed, is described as 

 smooth when no pits exist, Perischoechinoida, Cidaroida, or is described as having pits when 

 such structures exist, Centrechinoida (text-figs. 208-213). Each half-pyramid is surmounted 

 by an epiphysis, which is narrow, when it extends only slightly beyond the half-pyramid, or 

 wide, when it extends across the foramen magnum uniting in median suture with its fellow of 

 the opposite side (text-figs. 207-216, p. 184). Each epiphysis presents a glenoid cavity and 

 tubercles for articulation with the brace, and in the Camarodonta crests for support of the tooth 

 (Plate 2, fig. 14; Plate 5, figs. 2, 5, 9). The brace is a block-shaped plate and presents condyles 

 and foramina which interlock with the glenoid cavities and tubercles of the epiphyses (Plate 2, 

 figs. 9, 13, 15). In all regular Echini there are five compasses, each composed of tw r o pieces 

 joined by a transverse suture (Plate 2, fig. 12). The lantern muscles are termed protractors, 

 retractors, interpyramidal, brace muscles, circular compass and radial compass muscles (Plate 5, 

 figs. 1, 2, 4, 12; Plate 4, fig. 4; text-figs. 218-236, pp. 191, 193, 197). 



The perignathic girdle, when existent, consists of processes arising from the basicoronal 

 plates. Apophyses are upward growths, of basicoronal interambulacral plates; auricles are 

 here limited to separate pieces arising from the basicoronal ambulacral plates, to w r hich they 

 are joined by close suture, not being continuations of these plates as apophyses are of inter- 

 ambulacral plates (text-figs. 218-230, pp. 191, 193). The auricles in Phormosoma give rise 

 to radial somatic and radial peristomal muscles, which are known only in the Echinothuriidae 

 (text-fig. 226). 



The diameter of the test in regular circular Echini, where the axes are known, is measured 

 on the plane of the ambitus and passing through ambulacrum III and interambulacrum 5. 

 In regular Echini, which are elliptical in horizontal outline (Echinometridae) , or elongate 

 irregular Echini, the length is taken in the longer axis and includes the over-all measurement 

 of the corona. 



