182 ROBERT TRACY JACKSON ON ECHINI. 



(Plate 2, fig. 9), not pitted as in tiic Centrechinoida. The half-p3'ramid seen from within 

 shows a dental slide for the support of the tooth (Plate 13, fig. 14; Plate 50, fig. 12), as 

 in Recent Echini. The styloid processes do not extend above the base of the foramen 

 magnum, in this feature being like the Cidaroida, but unlike the Centrechinoida, where the 

 styloid processes are visible in face view (text-fig. 211). The epiphyses cap the half-pyra- 

 mids and are narrow, projecting only slightly beyond the limits of the outline of the half- 

 pyramids (text-figs. 207, 208; Plate 12, figs. 1, 7; Plate 27, figs. 4-6). Each epiphysis pos- 

 sesses a glenoid cavity for the interlocking of the brace (Plate 12, fig. 8; Plate 68, fig. 12). 

 The brace is a block-shaped plate, practically identical with that of living regular Echini. It 

 rests on the epiphyses, and its condyles fit into their glenoid cavities (Plate 12, figs. 3, 6, 7; 

 Plate 27, figs. 4-6). A compass is such a delicate structure that it is rarely preserved, but 

 one was found in Archaeocidaris (Plate 12, figs. 1, 2) resting on the brace and with the 

 typical bifid outer end. A compass is also shown in Pholidechinus (Plate 27, fig. 5), but the 

 bifid end is gone, or possibly it did not have such a bifurcation, as it is not existent in all 

 Recent compasses. These are the first cases of compasses described from Palaeozoic species. 



It is seen that in the Palaeozoic Echinocystoida and Perischoechinoida the lantern is 

 made up of forty pieces, as in Recent regular Echini. In structure it approaches closest to 

 that of Cidaris. There were doubtless the same sixty muscles as described in Strongj'locen- 

 trotus. It is interesting to see how near the characters of these ancient genera are to those of 

 living forms, and also that their closest comparison is with the young of the latter. 



In the Cidaroida the lantern is the most primitive of any Recent Echini. Loven (1892) 

 showed the character in the young of Goniocidaris ; here at a very early stage the pyramids 

 are very wide-angled, with long interpyramidal muscles and a deep foramen magnum (text- 

 fig. 209; Plate 2, fig. 17). In these characters it differs from the adult, but is practically identi- 

 cal with the Perischoechinoida. According to Loven, at this early stage the protractor and 

 retractor muscles are inserted directly on the primordial interambulacral plates, which are still 

 in place. This condition again by inference is close to that in the Palaeozoic genera where 

 (excepting Miocidaris) apophyses and auricles are apparently always absent, and the pro- 

 tractor and retractor muscles were without doubt attached directly to the basicoronal inter- 

 ambulacral plates (text-fig. 221, p. 193). 



Loven (1892) described the lantern and muscles of "Cidaris papillata" {Cidaris cidaris 

 (Linne)) with his usual care. To show another type and to bring out certain points, the lan- 

 tern of Eucidaris tribuloides is here shown in text-fig. 210 and Plate 2, figs. 7-16. In this species 

 the teeth are grooved and extend far beyond the pyramids ventrally (Plate 2, figs. 7, 8). The 

 tooth rests against dental slides on the inner face of the pyramids, but the styloid processes are 

 slight and do not extend above the foramen magnum so as to be seen in face view (Plate 2, fig. 

 10), as they do in the Centrechinoida. The pyramids are comparatively narrow-angled, stand- 



