1913-14.] Abnormal Echinoids in the Royal Scottish Museum. 245 
A general type of regulation occurs in the Echinus described by Ritchie 
and MTntosh (loc. cit.), where a simple increase in the length of the inter- 
ambulacral plates compensated for and bridged the potential gulf left by 
the disappearance of the ambulacrum. But here the mode of regulation 
is essentially different. It would seem that before the ambulacrum ceased 
to be formed some disturbance occurred in the growth area, for a couple of 
exceedingly large inter-ambulacral plates in proper series have been formed. 
Subsequently to the assumed damage the ocular plate was cast off or 
absorbed, and then a large growth area at the external margins of the two 
genital plates became continuous and gave rise to an enormous median, 
roughly triangular plate which succeeds the detached end of the ambul- 
acrum and terminates two half -rows of inter-ambulacral plates. The 
remaining space between this and the apical system is filled in, not by 
regular inter-ambulacral plates, but by a group of irregular casual plates, 
the group being roughly symmetrical about a median longitudinal axis. 
This type of regulation is a stage between the complete unharmed 
inter-ambulacral areas (exhibited in the Echinus esculentus described by 
Ritchie and MTntosh, or in areas 4 and 5 of the Amblypneustes recorded 
by Hawkins *), and the complete disappearance of a total ray, as occurs in 
the specimens recorded by Bell-)- and Philippi.!; 
Soft Parts. — So far as could be distinguished, the badly preserved 
genitalia presented the normal five-partite arrangement and contained 
male elements. 
(ii) Echinus esculentus, Linn. 
The specimen was obtained by Mr F. G. Pearcey in the Cromarty Firth 
at a depth between 8J to 16 J fathoms. It contained shrivelled female 
reproductive organs. Even in the dry condition in which it was preserved, 
when still covered with spines, it showed marked irregularity of outline. 
This in plan was trapezium-shaped. There was a distinct flattening of 
the test in the part which lay between the vertex and the long side of the 
trapezium, and the apical system was so distorted that it lay on this 
flattened surface, only one edge reaching up to the summit of the test. 
The maximum horizontal diameter of the test was 5 cm., its height 3 cm. 
There were the usual five teeth in Aristotle’s lantern. 
The spines having been removed, there was revealed the type of 
abnormality shown by the specimen of Amblypneustes described above, 
but in a more extreme degree; for here ambulacrum V. had almost dis- 
* Hawkins, Proc. Zool. Soc., 1909, part ii. p. 714, figs. 226-230. 
t Bell, Jour. Linn. Soc., vol. xv., 1881, p. 126, pi. v. 
\ Philippi, see Bateson, Materials for the Study of Variation , London, 1894, p. 443, fig. 137. 
