346 Herbert L. Hawkins — Studies on the Echinoidea. 



leaning outwards towards the interambulacrum, though practically- 

 vertical when viewed from the branchial incision. The process is 

 narrow at the base, and a little broader at the summit, where it 

 is capped by an ovoid expansion which overhangs its sides. It 

 leans against, and is visibly sutured to, a much more massive 

 buttress which is almost wholly interambulacral in position, and 

 constitutes the "dip-slope" of the whole structure. Loven 

 (" Echinologica ") seems to have ascribed this buttress to the 

 ambulacrum in its entirety, but this is contrary to my observations. 

 His meaning is rather difficult to follow in the passage (loc. cit., p. 51) 

 dealing with this structure. The buttress has a rounded crest, 

 which overhangs, in a slight eave, the concave hollowing of its 

 side adjacent to the branchial incision. The whole buttress, at first 

 directed in a line continuous with that of the process, gently curves 

 back towards the adradial suture, near to which, but on the 

 interambulacral side, it passes until it sinks to the general level of 

 the test. The position of this buttress is indicated externally by 

 the specialized smooth area referred to above (section 2, a). The 

 texture of the stereom of the buttress is different from that of the 

 process, the former being soft and friable, while the latter is almost 

 porcellanous. It is obvious that the buttress is no part of the 

 perignathic girdle strictly speaking, but is merely a strengthening 

 structure for the better support of the process, and the stiffening 

 of the extremely thin plates in the region of the peristomial 

 invagination. 



In the more important specimen, the diameter of which is 74 mm. 

 (the peristome being 12 mm. across), the average height of the 

 processes, measured from the internal surface of the test at their base, 

 is almost exactly 3 mm. The interradial buttresses extend towards 

 the ambitus for a distance of about 8 mm. from the apices of the 

 branchial incisions. 



The ambulacral pores pass between the bases of two processes 

 without perforating them and without being deflected from their 

 straight course. They pierce the test very obliquely near the 

 peristome, but otherwise show no disturbance due to the processes. 

 I have seen no trace of a suture at the base of any of the twenty 

 processes examined, but these lines are so faint on even recent tests 

 that there is no reason to doubt their occurrence. 



In the perradial line, just at the peristomial margin, there is 

 a small ovoid prominence or thickening of the invaginated edge of 

 the ambulacrum. This unpaired structure is probably not a part 

 of the girdle, in as far as that apparatus serves as a support for the 

 jaw-muscles. Of itself the prominence is of trifling importance, but 

 a similar thickening of much greater relative size occurs in Conulus, 

 so that its presence in Plesiechinus seems worth recording. 



(c) The interradial '■'■ridges". (See p. 344, Text-figs. 2, 4.) 



At first sight the perignathic girdle seems to be composed solely 

 of the ambulacral processes above described, the equivalents of the 

 ridges (Duncan) or apophyses (Jackson) being so feebly developed. 

 There is a slight thickening of the test along the margins of the 



