SYSTEMATIC CLASSIFICATION OF ECHINI. 219 



procumbent (text-figs. 214-216, p. 184) though inclined in Echinocyamus (Loven, 1892). The 

 teeth are keeled, foramen magnum shallow, epiphyses small and narrow, brace small, wanting 

 in Laganum (Loven, 1892), compasses wanting. The pyramids are more or less highly modi- 

 fied (least in Echinocyamus, Loven, 1892) with wing-like lateral expansions and without ridges 

 for the attachment of interpyramidal muscles. The perignathic girdle consists of auricles only, 

 which are situated on the ambulacral plates (text-fig. 231, p. 197) as in the Centrechinoida, 

 or may be transferred to the primordial interambulacral plates (text-fig. 236, p. 197), as shown 

 by Loven (1892). Each pair of auricles bears one or two pairs of retractor muscles, and in 

 addition one pair of protractors. This last is a peculiar feature of the group, as in all 

 other Echini with lanterns the protractors are inserted on the interambulacral plates, or on 

 apophyses arising from the same, but not on auricles. Respiration is apparently by ambu- 

 lacral gills only. Sphaeridia are situated in enclosed pits (Loven, 1874). 



The suborder Spatangina is characterized by tests which are conical, heart-shaped, or of 

 various forms, often extraordinary (Pourtalesia), but bilaterally symmetrical through III, 5. 

 The ambulacral plates are simple and the areas are usually petaloid dorsally. There may be 

 two pores or one to a plate, or plates may be in part imperforate. The plates may be low, or 

 high, and of very specialized form, especially in the bivium ventrally. Plates, especially in 

 ambulacrum III, may be high, nearly or quite hexagonal in form, with pores superposed in 

 the middle of the plate (text-fig. 8, p. 54), a feature not seen in the adults of other forms since 

 the Ordovician Bothriocidaris, though common in the young of regular Echini. The primordial 

 ambulacral and interambulacral plates are typically in the basicoronal row (Plate 3, fig. 15), 

 but, in the shifting of parts, one or the other may in part be pushed dorsally. The base of the 

 corona is apparently never resorbed by the advance of the peristome, which latter is usually 

 densely covered with small non-ambulacral plates (text-figs. 27-28, p. 70, and 54, p. 80). Ocular 

 plates may be all exsert, that is, do not reach the center line, or oculars I, V, or I, V, IV, or 

 I, V, IV, II may be insert (text-figs. 172-175, p. 149). In cei-tain specialized types, as seen 

 in some species of Pourtalesia (Loven, 1883; A. Agassiz, 1904), the oculars are apparently 

 partially or wholly absent in adults. Four genitals are typically present, but 5 is absent and 

 more genitals may be wanting. Genitals 2, 3 may be typically fused (Cystechinus, A. Agassiz, 

 1904), as seen in variants of regular Echini (p. 167). The madreporic pores are in genital 2 

 or extend to 3 where these two plates are fused. An extension to 3 is a coimnon variant in 

 some regular Echini (p. 172). Genital 2 may be much enlarged and extended posteriorly 

 so as to reach interambulacrum 5 (text-fig. 173, p. 149). The lantern is wanting in adults, but 

 occurs in young Echinoneus (pp. 189, 217), as shown recently by Mr. Agassiz (1909). Spines 

 are short and undifferentiated, or may be highly produced and with marked areal differentia- 

 tion. Tubercles in some genera are subject to much areal differentiation, making patterns 

 on the test. The primary tubercles in Metalia, Meoma, and Brissus are perforate and crenu- 



