8 Herbert L. Hawkins — Studies on the Ecliinoidea. 



plates, being crossed by transverse sutures, and they can be 

 distinguished from the actual elements of the girdle by their less 

 polished surfaces and open stereom-mesh in direct continuity 

 with that of the plates from which they rise. In Jurassic forms, 

 and apparently in their nearest relatives in the Cretaceous period, 

 the buttresses are short and steep ridges which radiate for a varying 

 distance near to or upon the adradial sutures. In Discoides the same 

 series of radiating ridges can be recognized, although they are 

 reinforced by others of0 similar trend and by a circular region of 

 elevation surrounding ttie peristome. In Coyiulus, where the whole 

 inner adoral surface of the interambulacral area is much thickened, 

 no separate buttresses can be distinguished; each area may, however, 

 be regarded as being lined by a fused and extended mass of 

 butti-esses. 



The apparently universal presence of this adambital support for 

 the processes is almost peculiar to the Holectypoida. The girdles of 

 the Cidaroida and Diademoida are sufficiently sti'ong in themselves 

 to stand without additional help-; those of the Clypeastroida, though 

 often relatively slender, are so encompassed by the ridges and pillars 

 which cross the test-cavity that they seem to be equally independent, 

 in most cases, of special buttresses. This contrast is suggestive of 

 some special relation between the perignathic girdle and the jaws in 

 the Holectypoida wherein they differ from the other gnathostomatous 

 Orders. Perhaps it may be connected with the undoubtedly "flaring " 

 character of the lantern. In the Regular Orders the jaws are almost 

 vertical, and their weight can best be supported by a "sling" of 

 muscles, on all of which the "pull" would be almost vertically 

 downwards. In the Clypeastroida the jaws are practically horizontal 

 in typical forms, and they almost articulate with the processes. The 

 onus of their support will be shared between the adoral surface of 

 the test and the processes, the strain on the latter acting again 

 A'ertically downwards. But with a lantern inclined at, say, 45° from 

 the vertical (and a girdle correspondingly splayed), the downward 

 weight would tell upon the lath-like processes obliquely, so that they 

 would require to be strengthened from below and without to prevent 

 fracture. In support of this suggestion it may be recalled" that in 

 those Clypeastroids in which the lantern is elevated above its normal 

 prostrate position, the processes have adambital keels similar in many 

 respects to those of Plesiechimis. 



Although the buttresses are obviously, and probably originally, 

 connected with the mechanism of the perignathic girdle, they assume 

 a more far-reaching function in the Cretaceous Holectypoida and 

 their Clypeastroid descendants. The adoral I'egions of the test in 

 both Orders is normally very thin, so that the peristomial invagina- 

 tion, as well as the sharply reflexed ambital margin, demand a 

 girder-like support. In the Jurassic forms, the latter line of 

 weakness is not seriously developed, and the buttresses in conse- 

 quence are restricted to the central part of the surface, in the region 

 of the invagination of the peristome. In Discoides the ambital 

 margin is angular rather than curved, so that the girders are extended 

 across that fragile zone. In Conuhis, although the ambitus may be 



