1913-14.] Abnormal Echinoids in the Royal Scottish Museum. 241 
XVIII.— Abnormal Echinoids in the Collection of the Royal 
Scottish Museum. By James Ritchie, M.A., D.Sc., Royal 
Scottish Museum ; and James A. Todd, M.A., B.Sc. Communi- 
cated by William Eagle Clarke. (With a Plate.) 
(MS. received May 16, 1914. Read June 1, 1914.) 
CONTENTS. 
PAGE 
I. Introductory Notes on Regulation, Duplication op Parts, Relation op 
Ocular Plates to Coronal Growth, and Condition op Specimens . 241 
II. Examples op Incomplete Development ..... 243 
(i) Amblypneustes ovum. 
(ii) Echinus esculentus. 
III. Total Variation prom Five to Six-rayed Form — Echinus esculentus . 247 
IV. Explanation op Plate ........ 252 
I. 
» 
“ Echini are a particularly good group in which to study questions of 
variation, because here variations can usually be expressed in very 
definite terms of numerical or other equally positive character.” * On 
this account, and because, in spite of much description, the variants liable 
to occur in sea urchins have not yet been exhausted, the three specimens 
described below are recorded. Each of these exhibits a pronounced 
abnormality in the major symmetries. Two of them resemble another 
abnormal echinoid in the same collections, already discussed in Proc. Zool. 
Soc ., f in lacking part of a definite ambulacrum ; but the means by which 
the tests have accommodated themselves to changed conditions of growth 
differ markedly in each of the three cases. The third specimen exhibits, in 
place of the normal five-radiate arrangement, almost perfect hexamery — 
a type of abnormality very different from that of the first two specimens. 
For in these the distortion is due to incomplete development caused by 
interference with the processes of growth, while there the hexamery is a 
fundamental change in symmetry, is congenital in origin, and probably 
represents the type of variation known as duplication of parts. 
* R. T. Jackson, “ Phvlogeny of the Echini, with a Revision of Palaeozoic Species, 5 ’ in 
Mem. Boston Soc. Nat. Hist., vol. vii., 1912, p. 51. 
t Ritchie and MTntosh, Proc. Zool. Soc., 1908, p. 646. 
VOL. XXXIV. 
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