260 ROBERT TRACY JACKSON ON ECHINI. 



History; specimens from the same locality in the Museum of Comparative Zoology ; F. Springer 

 Collection; (?) Lower Carboniferous, Eureka District, Nevada (Walcott). 



The type specimen in the American Museum of Natural History is one of the most nearly 

 complete specimens known in the genus, and measures 53 mm. in diameter through J, E (Plate 

 8, figs. 5, 6; Plate 9, figs. 6-8). It is not quite complete in the basicoronal row, but excepting 

 area I is very nearly so. In areas A and C, in the basicoronal row, the plates of columns 1 and 4 

 are nearly whole, whereas the plates of columns 3 and 2 are small, representing only the dorsal 

 part of each plate, of which the ventral portion has been resorbed. In areas E, G, and probably 

 I, in the basicoronal row, the plates of columns 1 and 3 are small, of which the greater part has 

 been resorbed, whereas the basicoronal plates of columns 4 and 2 are nearly complete plates 

 with slight ventral resorption. A similar condition is shown in A . rossica (text-fig. 239 bis, p. 264) , 

 and in part in A. urii (Plate 15, fig. 1). This alternation of large and small plates in the basi- 

 coronal row is due to the cutting off of the ventral part of the plates by resorption in the advance 

 of the peristome (text-fig. 26, p. 70). The relative position of the large and small plates differ- 

 ing by the two arrangements I earlier ascribed (1896, p. 219) to the relative position of the in- 

 troduction of column 4, as seen in the restoration (Plate 9, fig. 8). By restoring schematically 

 the plates that have been resorbed, we see that if column 4 is introduced to the right of the 

 center, then in the basicoronal row the plates of columns 1 and 4 are large; or if column 4 is in- 

 troduced to the left of the center, then in the basicoronal row the plates of columns 1 and 3 are 

 small. There are no gill-slits on the peristomal border (p. 223). 



The peristome in this fine specimen is the most nearly complete one known in the genus. 

 There are two columns of ambulacral plates in an area, doubtless formed by flowing down from 

 the corona as in Cidaris (text-figs. 46, 47, p. 80). There are also many irregular scale-like 

 non-ambulacral plates imbricating strongly. These plates, it is believed, are formed in place, 

 and are not derived from the corona, as discussed under consideration of the peristome (p. 

 86). It was in this specimen that secondary spines were first shown in the genus. A complete 

 lantern is not known, but the essential parts are shown in Plate 8, fig. 6, and Plate 9, fig. 10. 

 The pyramid is wide-angled with rather deep foramen, epiphyses narrow, capping the half- 

 pyramids. 



*Archaeocidaris legrandensis Miller and Gurley. 



Plate 8, figs. 7, 8; Plate 9, figs. 12, 13; Plate 10, figs. 3, 4. 



Archacocidaris legranch'iisis Miller and Gurley, 1SS9, p. 373, Plate 10, fig. 1.5; Keyes, 189.5, p. 185; 



Klem, 1904, p. 50; Lambert and Thiery, 1910, p. 124. 

 Eoeidaris blairi Miller, 1892, p. 683, Plate 22, fig. 1; Klem, 1904, p. 67. 



Test in side view (Plate 8, fig. 8) is only moderately compressed, and measures 26 mm. in 

 diameter, and the ventral view (Plate 8, fig. 7) is of about the same size. The ambulacral 

 plates are not preserved, but their position is located by the outlines of the adambulacrals. 



