1908.] AN ABNORMAL ECHINUS. 657 



We can find no later reference to new descriptions of abnor- 

 malities of a similar nature in major symmetries, although two 

 early cases are mentioned by Gauthier * which Bateson appears to 

 have omitted. Tlie first, a specimen of Echinohrissus orbicularis, 

 is described by Cotteaut as having the anterior ambulacrum 

 completely atrophied ; the second, a Pyrina ovulum, in which the 

 right posterior ambulacrum was wanting, has also been described 

 by CotteauJ. 



The majority of the cases cited belong to fossil forms, this 

 being no doubt due to the greater readiness with which an 

 abnormality may be detected in a clean fossil test than in a 

 i-ecent well-preserved specimen, where plate-groupings are 

 obscured by epiderm and spines. The cases to which the present 

 example bears closest resemblance are those grouped by Bateson 

 in his class (2), wherein the specimens are distinguished by 

 the " partial or total disappearance of a definite ambulacrum or 

 interambulacrum." At first glance the parallel between the 

 Shetland specimen and the Echinus melo described by Philippi § 

 appears to be almost complete, but in that case, as in the specimens 

 described by Bell ||, Ohadwick^I, and Osborne**, the defaulting 

 member constitutes a complete morphological system, the homo- 

 logue of an Asteroid ray, whereas in the present specimen only 

 the ambulacral portion of a ray has sufiered reduction. There 

 is a much closer resemblance to specimens of Hemiaster, described 

 by Gauthier ft, in which only the ambulacral portion of a ray has 

 disappeared. Of those specimens the case of Hemiaster batnensis. 

 No. I, appears to show the closest analogy. There the corre- 

 sponding ambulacrum, the left postei-ior, has partly JJ disappeared, 

 having at a certain stage received a check in development^ 

 the stage being indicated by the dying out of the ambulacral 

 pores and by a slight depi-ession in the test. As a con- 

 sequence four sets of interambulacral plates follow one another 

 without interruption, and the posterior interambulacral suture 



* Gauthier, M. V., " Sur quelques Echinides monstrueux appartenant au genre 

 Semiaster." C. R. Assoc. Franc, pour I'avanc. des Sciences, 13th. Sess., 1884 

 (Paris, 1885), p. 259. 



t Cotteau, G., Echinides nouveaux ou peu connus, 1862, p. 66, pi. ix. 



X Cotteau, G., I. c, 1867, p. 133, pi. xviii. 



§ Philippi, P. W., Arch, fiir Naturg. iii, 1837, p. 241, and plate. 



II Bell, P. Jeffrey, Journ. Linn. Soc. (Zool.), xv. 1881, p. 126, and plate. 



'11' Cliadwick, H. C., " Note on a Tetramerous specimen of JSchinus esculentus ". 

 Trans. Liverpool Biol. Soc, 1898, p. 288, and plate. 



** Osborne, H. L., " A case of variation in the number of ambulacral systems of 

 Arhacia punctulata " : American Naturalist, vol. xxxii. 1898, p. 259, and figs. (The 

 reference to Lang's ' Comparative Anatomj^' vol. ii, p. 321, is evidently a slin for 

 p. 341.) 



ft Gauthier, M. V., 1885, I. c, p. 258, and plate. 



Xt Bateson seems to err in placing this example among those in which " one 

 ambulacrum is wholly wanting in the affected radius " (7. c. p. 443) ; for while the 

 functional part of the ambulacrum is not represented on the test as found, yet in 

 the earlier stages of development the ambulacrum apparently did exist, for beyond 

 the point where the ambulacral groove ought to run " appai-aissent quelques paires 

 de pores arrondis, qui continuent I'aire ambulacraire de I'autre c6t(5 [that is, the 

 oral side] du fascicle." Gauthier, I.e., p. 259. 



42* 



