396 Herbert L. Hawkins — Studies on the Echinoidea. 



divorced from the true girdle as regards function. That it serves as 

 a useful support for the very slender processes is ohvious ; and that 

 it consists of the bevelled and otherwise modified edges of the very 

 thick proximal coronal plates seems equally clear. In the previous 

 article in this series (Geol. Mag., August, 1917), I showed that the 

 processes of Plesiechinus are supported by thick buttresses which are 

 built almost wholly of thickened interambulacral plates. The 

 buttresses of the Jurassic genus are so similar in character, though 

 far different in extent, to the carinate thickenings of the adoral 

 surface of Discoides, that it is practically certain that the latter 

 structures represent a phylogenetically later stage of the former. 

 The buttresses of Plesiechinus pass towards the median interradial 

 lines after leaving the processes, and subsequently return to the neigh- 

 bourhood of the adradial sutures. In Discoides (see PI. XXV, Fig. 3) 

 they may be said to have a similar course, but each pair converges 

 interradially to such a degree that fusion results. Thus the whole 

 proximal part of each interambulacrum becomes much thickened, 

 although elsewhere on the adoral surface the test is extremely thin. 

 The false ridge therefore represents the bevelled edge of the fused 

 buttresses. 



Nevertheless, the false ridge is a more specialized structure than 

 the preceding conclusion would demand. It is of closer texture 

 than the ordinary parts of the buttresses, projects to a varying 

 distance above them, and is hollowed in the centre. Clearly it 

 must have served some special function, for which these characters 

 are adaptations. 



The strongly inclined (almost horizontal) pyramids of the lantern 

 of Clypeaster actually articulate with the processes. There are no 

 buttresses or analogous supports in the immediate vicinity of the 

 peristome in this type. The lantern of Discoides, as Loven and 

 I have described it, was evidently "flaring" in character, though 

 far more nearly vertical than is the case in Clypeaster. Since 

 pyramids are interradial in position, it follows that they would lean 

 over the proximal interradial plates, if splayed outwards. In 

 Discoides these plates are much thickened ; hence they must be 

 bevelled off so as to afford play for the pyramids. It is at least 

 likely, by comparison with the closely related Clypeastroida, that 

 the pyramids may have actually leaned against the bevelled edges 

 (false ridges) when the jaws were closed. This would be a preliminary 

 to the actual articulation found in the last-named group. It may 

 therefore be suggested that the smooth, concave surfaces of the false 

 ridges served as "slides" for the pyramids, allowing them to be 

 drawn up or down according as the teeth were withdrawn or 

 extruded ; and preventing the strain which would inevitably fall 

 upon the muscles holding an inclined lantern together. The 

 hollowing of the adoral faces of the false ridges would, as shown 

 above, allow freedom of action for the protractor muscles. 



3. Summary. 



The conclusions of Loven as to the composition of the perignathic 

 girdle of D. cylindricus are confirmed in essential particulars, while 



