212 R. T. JACKSON— STUDIES OF PAL.EECHINOIDEA. 



noderms. Its nearest analogy perhaps is seen in Cidaris (plate 8, figure 

 48), where the pore pairs in the peristome are arranged vertically, while 

 the pore pairs of the corona are horizontal in position. Jaekel (20) states 

 that the pores in Bothriocidaris are arranged vertically in all coronal am- 

 bulacral plates, though this observation is contrary to Schmidt's (37) (see 

 our figure 4, page 234). In the ventral portion of the specimen of 

 Pholido cidaris are small ovally rounded ambulacral plates which are dis- 

 sociated, but apparently belong to the peristome. No spine tubercles 

 were observed on any ambulacral plates, but their absence is probably 

 due to the weathering of the surface.* 



The interambulacrum of Pholidocidaris meeki (plate 9, figure 54) orig- 

 inates ventrally in a single pentagonal plate, 1', like Lcpidechinus (plate 7, 

 figure 42), Goniocidaris (figure 3, page 234), Strong ylocentrotus (plate 3, 

 figure 8), and other genera. That this first plate is the ventral border of 

 the corona we know from the presence of a certain number of buccal pyra- 

 mids, D, in place and closely related to the corona. In the second row 

 there are two plates, 1 and 2, which lead up by additions to the two col- 

 umns of adambulacrals,t ^s shown in many other genera in this paper. 

 The third, fourth, fifth and sixth columns of interambulacral plates are 

 introduced in succession, as indicated by the numbers and dotted lines, 

 much as in Melonites (plate 2, figure 2), allowing, however, for the differ- 

 ences in their regular imbricating plates of Pholidocidaris. 



Comparing the interambulacrum of Pholidocidaris (plate 9, figure 54) 

 with that of Lepidocentrus (figure 2, page 223) and Lepidechinus (plate 7, 

 figure 42) one striking feature is noticeable, namely, that these two last 

 mentioned genera have a very accelerated development, the columns of 

 plates being introduced much more rapidh^ than in Pholidocidaris. It is 

 also noticed that the form of individual plates, while irregular in outline 

 in all three genera, is also diff'erent in the three genera. 



The interambulacral plates of Pholidocidaris irregularis as figured, have 

 relatively large primary spine bosses, ])ut these are situated irregularly 

 and not in the center of the plates, as is usual in primary tubercles. In 

 addition to these larger bosses, numerous small bosses are scattered thickly 

 over the surface of tlie plates. In the present species, P. meeki, the sur- 

 faces of the plates are too much worn to show any spine tubercles, ex- 

 cept a few of the primary size (none are shown in the figure). These are 

 similar in position to those in P. irregularis^ and probably the spine tuber- 

 cles of both species are alike. The spines are tapering, acicular, termi- 



* On account of imperfections of the specimen, to avoid confusion only the clearest portions are 

 shown in plate 9, figure 54. Some of the details described do not appear in the figure. 



t Column 2 is wanting for a space above the second plate of its series, but the place where absent 

 plates should exist is indicated by the dotted line. A misplaced ambulacral plate, A, lies in this 

 line where interambulacra should be. 



