1913-14.] Abnormal Echinoids in the Royal Scottish Museum. 251 
p. 446). Stewart has described an example of Amblypneustcs griseus, in 
which an ambulacrum only was imperfectly duplicated ; Cotteau a specimen 
of Hemiaster batnensis, in which an ambulacrum only was completely 
duplicated ; Gautier an Hemiaster latigrunda, in which an ambulacrum 
was completely duplicated and an inter-ambulacrum imperfectly ; and in 
the present Echinus esculentus both ambulacral and inter-ambulacral series 
show perfect duplication. 
Along with our Echinus must be reckoned the specimens of Tripneustes 
esculentus and Strongylocentrotus drobachiensis (two cases) described by 
Jackson; for in all of these, two of the six genital plates are adherent, 
apparently indicating again the residue of the almost perfect duplication 
of a ray. Regarding this curious phenomenon of two fused genitals with 
an ocular between them which he found in the only cases (three in number) 
of complete hexamery discovered amongst 50,000 sea urchins, Jackson 
says : “ It is certainly most extraordinary that this parallel structure 
should exist in three specimens, and indicates what I have elsewhere 
pointed out, how very definite extremely rare variation may be.” It adds 
to the wonder, and to the evidence of definiteness of particular variations, 
that the specimen above described, belonging to still a different genus from 
Jackson’s, should repeat for a fourth time in hexamerous Echini this 
curious abnormality. 
The evidence as to means of growth-compensation controverts the find- 
ings of Jackson, who found that £ ‘ in the six-rayed specimen [. Tripneustes 
but also in his other specimens] evidently the space gained to add the extra 
ambulacrum and inter-ambulacrum is attained by building ambulacra of 
practically the usual width, but narrowing all the inter-ambulacra equally 
to much less than the usual width. This emphasises the conclusion 
gathered from normal Echini that the inter-ambulacrum is essentially a 
space-filler and adapts itself to fill what space is available between the 
ambulacra which are the most essential structures.” In view of these 
statements the six-rayed Echinus was compared, as to relative proportions 
of ambulacra and inter-ambulacra, with a normal specimen of, as nearly as 
possible, the same size, with these results : — 
Circumference 
at Ambitus. 
Average Width 
of Inter-ambu- 
lacral Areas. 
Average Width 
of Ambulacral 
Areas. 
Normal Echinus 
Six-rayed Echinus . 
318 mm. 
314 mm. 
41 mm. 
33 mm. 
23 mm. 
19 mm. 
