234 



R. T. JACKSON — STUDIES OF PAL^ECHINOIDEA. 



genera.'^ In this figure of Bothriocldar is the two pores in the ambulacral 

 plates are arranged in a horizontal plane. This arrangement, according 

 to Jack el (20), is incorrect, and he figures a specimen with the pores 

 lying in a vertical plane. Jack el's figure would not serve my purpose, 

 so 1 copied Schmidt's, and of colirse could not properly alter the position 

 of the pores. 



In Goniocidaris (figure 3) there is a single row of 10 ambulacral plates 

 around the mouth instead of 2 rows, as in Bothriocldar is (figure 4). Suc- 

 ceeding this row there is a row of 10 ambulacral and 5 interambulacral 

 plates, as in Bothriocidaris. In the third and succeeding rows of Gonioci- 

 daris there are 10 interambulacral plates (figure 5) instead of 5, as in 

 Bothriocidaris. In adult Goniocidaris and other members of its family 

 the ambulacral plates are low, with 2 pores, and many plates impinge on 



FiGrRE 3. — 'i'ontig Goniocidaris canalicitlafa, A. Ag., after Loven {27). Magnified 27 diameters 

 Figure 4. — Adult Bothriocidaris pahleni, Schmidt, after Schmidt [s7).\ 

 Figure 5. — Side view of young Goniocidaris, the same as figure 3. 



a, b, columns of ambulacral plates; i', first formed interambulacral plate; i, 2, further added 

 plates and columns of the interambulacra. 



a single interambulacral plate (plate 8, figures 47, 48) ; also the inter- 

 ambulacral plates of the adult are nearly or quite pentagonal in form. 

 In the young (figure 5), on the contrary, the ambulacral plates are nearly 

 as high as the interambulacral ; further they are hexagonal and have but 

 a single pore. The interambulacral plates of the 3'oung are hexagonal 

 (figure 5) instead of i^entagonal. The relative form of the plates in 3^oung 

 Goniocidaris is almost exactly the same as in the primitive type, Bothrio- 

 cidaris (figure 4). The Cidaridse are practically unchanged in essential 

 features from their earliest appearance in the Permian to the present 

 da3^ With the present understanding of the group, this family, as 

 evinced by the structure and development of the corona (as shown in 

 Loven's figures), is the simplest, least differentiated member of the Echini 

 excepting Bothriocidaris. 



* Except Tiarechinus, which has 3 plates in the second row. 

 t This figure is a combination of his figures la and \c. 



