STRUCTURE OF LEPIDOCIDARIS SQUAMOSUS. 221 



his " Synopsis of Paleozoic Echinoids," he includes it"under the genus 

 Lepidechinus. There are essential differences in the form of the interambu- 

 lacral plates, the ornamentation of the same, and the arrangement and 

 form of the ambulacral plates from Lepidechinus rarispinus^ Hall, as I can 

 safely assert, having studied Hall's types and also other specimens of the 

 species (plate 7, figure 42). Lepidocidaris squamosus also differs essentially 

 from Lepidechinus imbricatus, which is the type of the genus, judging from 

 Professor Hall's description, although unfortunately the species has never 

 been figured. 



In Meek and Worthen's figure of Lepidocidaris squamosus the axes were 

 incorrectly oriented, as they had nothing to guide them in fixing axes 

 but details of plate arrangement, and this method of orientation was un- 

 known. In plate 7, figure 41, the orientation is corrected. The interam- 

 bulacral plates of Lepidocidaris squamosus are comparatively thin and the 

 imbrication is slight. 



The ambulacral plates of Lepidocidaris squamosus are small, low, and 

 above the ventral border fail in many cases to pass across the half-area to 

 which they belong. This feature is a distinction from either Archdeocidaris 

 or Lepidechinus, and is similar in method, though not quite so complete 

 in degree, as in Palasechinus, as recently emended by Dr Duncan (8) and 

 shown in P. gigas (plate 7, figure 39). It also reminds one strongly of 

 the condition seen in developing Oligoporus (plate 6, figure 25). 



At the ventral border of the interambulacrum, as far as preserved (plate 

 7, figure 41), there are 5 columns of plates, although their outlines are 

 somewhat indistinct. The sixth column is introduced by a pentagonal 

 plate 6. This column has 2 columns on the left and 3 on the right at its 

 point of origin. No heptagonal plate bordering pentagon 6 can be made 

 out on account of the imperfections at that area. In the third row above 

 pentagon 6, the seventh column is introduced by the pentagon 7 with a 

 heptagonal plate, LI, on its left and 3 columns of plates on either side at 

 the point of origin. The eighth and last column added, as far as the 

 specimen shows, originates in pentagon 8, in the second row above penta- 

 gon 7. This eighth column at its origin has 3 columns on the left and 4 

 on the right. A heptagonal plate, H, lies on the right of the terminal 

 pentagon. This is the only entire specimen known, so that we cannot 

 tell whether more columns were added or not. From the contour of the 

 specimen it appears that its dorsal portion could not have reached much 

 more than to the ambitus ; therefore it is possible that more columns of 

 plates will be found to be characteristic of the species if more perfect 

 material is found. 



Lepidocidaris is evidently a near ally of Archseocidaris, and it is inter- 

 esting to find that the plate arrangement traced in the Melonitidse can be 



