56 



Mr. F. A. Bather on EociJaris and 



According to Spandel, tlie ridf^e .serves as a stop (" \Vi<ler- 

 lager ") lor the adjouiing plate; but his view is inconsistent 

 with the existence of a ridge on Loth upper and lower 

 margins. There is more probability in the opinion expressed 

 by Tornquist (1896), in reference to a similar structure in 

 Echinocrxnus, that the ridge merely marks a groove for the 

 attachment of the uniting ligament. On this view, the 

 presence of a ridge would indicate greater rather than less 

 flexibility. The interambulacrals of Cidaris heyserJ'uifji 

 must, however, have been united somewhat firmly, since 

 large portions of ambulacra are more common than isolated 

 ambulacrals, and, except on the adradial margins, they are 

 bounded by fractures rather than by sutural surfaces. 



The same fragment (E 11 21 J that shows the denticulation 

 so ]jbiinly also bears witness to considerable thickening of 

 the interanibulacrum towards the peristome and to the 

 existence of an internal prominence on each side of the 

 interanibulacrum, for the attachment of the jaw-muscles. 

 It is curious that no remains of the jaw-apparatus, which 

 this species certainly possessed, have yet been recorded. 



In reference to the structure of the main tubercle, Kolesch 

 criticises Geinitz and is in turn criticised by Spandel. 

 Briefly put, tlie mamelon is perfoiate, depressed hemi- 

 spherical, with slightly undercut neck, supported on a flush 

 platform of nearly twice the diameter of the neck, having 

 well-marked crenellae, of which the number rises to thirteen 

 in the larger ambital plates; fiom this the boss slopes with 

 slightly concave curve to the depressed definite scrobicule, 

 and, while generally smooth, is occasionally marked by 

 slight folds apparently continuous with the crenella:;. The 

 scrobicule is not so much a transver.se ellipse as a circle, 

 which is truncate above and below where contiguous or, as 

 in younger individuals and plates, confluent with the adjacent 

 scrobicules. 1 here is no definite ring of scrobicular tubercles, 

 but the extra-scrobiculur surface is covered with close-set. 



I 



