Thos, Roberts — Cretaceous EcJiinoids. 



117 



The parts missing from the test are those which lie on the left 

 side of a line drawn through the middle of the anterior single 

 ambulacrum and the posterior unpaired interambulacrum ; but it 

 is not possible to ascertain which of the paired areas of this side 

 that are wanting, as the pores in the ambulacral plates round the 

 peristome are indistinctly shown. 



Meyer figures' an internal cast of Galerites albogaJeriis with six 

 perfect ambulacral areas ; but I am not aware of any abnormal 

 example of Edtinoconiis subrotundus having been hitherto described. 



Meyer also figures and describes a specimen of Cidaris coronata,^ 

 in which a complete ambulacral and interambulacral area is wanting; 

 but the apical disc is not preserved, and I am unable to find out from 

 his figure and description which of the paired or unpaired areas that 

 were not developed. 



In 1881 Prof. Jeffrey Bell described and figured^ an abnormal 

 specimen of Amblypneustes formosus, in which the abactinal surface 

 shows a tetramerous arrangement of parts, but indications of 

 pentamerism are shown on the actinal I'egion. The apical disc, 

 however, is normal, i.e. it is composed of ten plates. As in tiie 

 Ecliinoconus under consideration, it is the left side of the test which 

 has undergone change. 



A somewhat similar abnormality, i.e. tetramerous abactinal and 

 pentamerous actinal surfaces, has been described* by Philippi in 

 Echinus melo, but in this case the apical disc was composed of four 

 basals and four radials. Philippi and Bell state ^ that it is the left 



Apical disc of Echinoconus subrotundatus from Lower Chalk, Shudy Camps. 

 aa, anti ambulacrum ; mp, madreporite plate ; ai, anal interradius. 



anterior area whicli has sufi"ered change ; but if the figure is correctly 

 drawn, it is certainly the right posterior paired ambulacral area 

 which, has been only partly developed. 



Mr. F. A. Bather ^ has receritly described a tetramerous Crinoid, and 

 he gives references to numerous examples of abnormality amongst 

 the different groups of the Echinodermata. 



^ Nova Acta Physico-Medica Leop. Car. Acad. 18. 1 (1836), Tab. xiii. figs ® 7. 



2 Ibid. p. 287, Tab. xiii. figs. 1-5. 



3 Journ. Lin. Soc. (Zoolo2;y), vol. xv. (1881), p. 126, pi. v. figs. 1-3. 

 * Arch, fiir Naturges. iii."(1837) p. 241, pi. v. 



5 Ibid. p. 242 and loc. cit. p. 127. ® Q.J.G.S. vol. xlv. p. 149. 



