DESCRIPTION OF PHOLIDOCIDARIS MEEKI. 213 



nating somewhat bluntly distally (probabl}' from erosion) and proximally 

 enlarged, as usual in echinoid spines. 



The oral region of the specimen of Pholidocidaris meeki is difficult to 

 make out, being considerably crushed and the parts not being sharply 

 differentiated. Buccal pyramids are to be made out in two areas, one of 

 which is shown in the figure. The pyramids are in two halves, united 

 b}'' a median suture, as in Archseocidaris (plate 8, figure 43), Melonites and 

 modern Echini. They lie opposite the interambulacral areas, their cor- 

 rect position. The teeth, which should occur at the oral apex at the con- 

 fluence of each pair of pyramids, are not preserved. Actual teeth have 

 not been described, so far as I am aware, in any Paleozoic Echini, though 

 the pyramids are known in several types. A very perfectly preserved 

 specimen of an Aristotles lantern in Yale University Museum shows the 

 pyramids with teeth in place in their usual position. This specimen, 

 which at present is not geherically placed, is from the Keokuk group, 

 Subcarboniferous, of Crawfordsville, Indiana. 



Pholidocidaris meeki differs from the only other known species of the 

 genus P. irregularis, in having more definite form to the interambulacral 

 plates, they being more rounded and almost biscuit-shaped ; also larger 

 in the latter species. In P. meeki no very small ambulacral plates were 

 seen in the corona, such as are figured in P. irregularis. The spines of 

 both species are much alike in size and shape. 



Meek and Worthen (31) expressed considerable doubt as to the affini- 

 ties of Pholidocidaris ; but, with this fuller knowledge of the genus, we 

 can have no hesitation in including it in the Lepidesthidse as an aberrant 

 genus. It has many features linking it with Lepidesthes, and yet not 

 perhaps sufficient differences to separate it off as a distinct but allied 

 family, which would be the only other alternative. 



Studies of the ARCHiEOCiDARiD^. ^ 



STRUCTURE AND PLATE ARRANGEMENT OF ARCHMOCIDARIS WORTHENI AND 



OTHER SPECIES. 



The species of Archseocidaris have for the most part been described 

 from dissociated plates and spines, the genus having been based by 

 M'Coy on such material. At least three species, however, have been 

 based on more perfect material. The type of Archseocidaris agassizii, Hall, 

 is in the Worthen collection at the University of Illinois, at Urbana, 

 Illinois (see foot-note, page 136). This specimen is from the Burlington 

 limestone, and as figured consists of a fragment of an interambulacrum 

 with attached spines. The spines lie thickly over the test so as to hide 

 most of the plates. In 6 specimens of this species which are in the Mu- 

 seum of Comparative Zoology (catalogue numbers 3032 to 3039) the 



