SEA-URCHINS. 477 



pRi'iguatluc gii'dle on the adoral sui-face, tliere are gvoove.s 

 (representing ridges in tlie test) that pass from the processes 

 alongside the ambulacra (but situated on the interambulacra) for 

 a short distance. The structure might be regarded as indicating 

 a gnidual rise of the inner surface of the test to form a keel which 

 culminates in the perignathic pi'ooess. As, among Regular 

 Ecihinoids, the perignathic girdle rises quite abruptly from the 

 inner surface, this gradual rise of the test towards the processes 

 shows a new feature, the beginning of the inner buttressing of 

 the test. 



In Holectypus, as the name implies, a diagnostic feature of the 

 genus as first tentatively suggested by Desor (11) is the absence 

 of grooves in the internal moulds. Tiiis of course means the 

 absence of internal ridges passing radially outwards to a point 

 beyond the ambitus. An investigation of some siliceous moulds 

 of H. ? sarthacensis has shown me that, although there is nothing 

 in the interambulacra to compai-e with the strong " cloisons " of 

 the succeeding genus, there nevertheless exists a considerable 

 thickening of the adoral regions of those areas, even more than in 

 Pjjgaster. In Dlscoidea the first signs of a really efficient internal 

 buttressing appear. Down a line, rather to the adradial side of 

 each half-interradius, there passes a thickening of the test which 

 is rounded near the peristome and becomes oarinate further out, 

 and which extends beyond the ambitus. The jxirtitions do not 

 pass for any considerable distance up the adapical surface. The 

 perignathic girdle tends to lean against the adoral' ends of the 

 supports. 



In Conulus no such well-marked buttresses appear, but the 

 interambulacral areas undergo a great amount of thickening 

 towards the peristome. Indeed, the perignathic girdle, which is 

 itself well developed, is often less internally elevated than the 

 interambulacral plates against which it reclines. The ambulacra 

 pass in sunken grooves across the adoral surface. In one specimen, 

 on cutting a section through the interambulacrum at a point just 

 outside the perignathic girdle, I found a large hollow to be included 

 between an inner and an outer wall of calcite. I have not been 

 able to verify the occurrence of this featui-e in other sections. It 

 may, therefore, have been an abnoinnality or the result of an 

 accident ; but if it should be found to be a genei'al tendency, or 

 even one of fairly frequent occurrence, it would be very significant 

 in the compaiisons that might be drawn between it and the double 

 flooring of the test of many of the higher Clypeastroida. 



It is only in Discoidea that the buttresses are in such a freely 

 projecting condition that they could be expected, by a growth in 

 their height and an accompanying depression of the adapical 

 sui'face, to form complete vertical partitions in the test. As it is, 

 these '■'■cloisons'' of Discoidea are rather more strongly developed 

 than the corresponding structures of Ecliinocifcimus, which other- 

 wise they resemble very clasely. In fact, (Gregory (50), in renaming- 

 the " genus " ealled Echinites by Duncan (44), which included only 



