242 
Proceedings of the Royal Society of Edinburgh. [Sess. 
Before proceeding to the description of these specimens, we would draw 
attention to a few facts of more general interest. 
I. Regulation. — It is clear that where so large an area as an ambul- 
acrum or inter-ambulacrum ceases to grow, after temporary development, 
much rearrangement in plates is necessary in order that the potential gulf 
in the test should be spanned. Two distinct modes of regulation occur. 
In the first place, the remaining plates in the neighbourhood of the 
abnormality may themselves wholly compensate for the deficiency by 
abnormal development in length and breadth. Such is the case in the 
Echinus esculentus already referred to, where, an ambulacrum only having 
dropped out, the two inter-ambulacral series of areas 4 and 5 have, by orderly 
increase, shared in filling in the space ; * or in Philippi’s case of Echinus 
melo ,*j* where the place of an ambulacrum with its associated inter-ambul- 
acral series ( i.e . a ray) is taken by a single inter-ambulacral series from each 
of areas 1 and 5. 
Or, in the second place, orderly growth of normal series of plates may 
be replaced or supplemented by the addition of abnormal plates varying 
much in shape and size. These are sometimes arranged with an approach 
to bilateral symmetry, as in inter-ambulacral areas 2 and 3 of the Ambly- 
pneustes described by Hawkins, J and as in the Amblypneustes described 
below (text-fig. 1), or they may form an irregular medley, as in the 
specimen of Echinus esculentus here described (Plate, fig. 1). 
It seems to be a general rule, however, that neither of these modes of 
regulation altogether compensates for the primary disturbance, for in every 
case growth seems to have been retarded, and the abnormal area is indicated 
by a depression in the test and occasionally by a marked distortion of the 
apical area from its normal position. 
II. Duplication of Parts. — Various stages of the phenomenon of duplica- 
tion have been described in Echinoids (see p. 250). These have represented 
duplicity only in partial degree, but it is possible that the hexamerous 
Echinus described below exhibits almost perfect duplication of both 
ambulacral and inter-ambulacral areas, and so completes the series of 
duplication stages. 
III. Ocular Plates and Coronal Growth. — It is generally held that the 
growth of new plates in ambulacral and inter-ambulacral series proceeds 
from the oculars. The case is stated strongly by Lambert. § 
* Proc. Zool. Soc ., 1908, p. 646. 
+ Archiv f. Naturg ., iii. p. 241, pi. 
f Proc. Zool. Soc., 1909, text-fig. 227. 
§ Lambert, “Note sur un cas de monstrosite de l’apex chez YEchinocorys vulgaris ” 
Bull. Soc. Yonne , 1890, xliv. 
