244 
Proceedings of the Royal Society of Edinburgh. [Sess. 
horizontal diameter was 3’7 cm., its height 3*2 cm. It possesses five am- 
bulacral and five inter-ambulacral areas, but ambulacrum IV. (Loven’s nota- 
tion) has its apical extremity separated from the apical system by about 
1 cm., and possesses no corresponding ocular plate. The space intervening 
between the apex of the incomplete ambulacrum and the apical system is 
filled in by an aggregation of modified inter-ambulacral plates, more or 
less irregularly arranged, but with each individual plate approximately 
symmetrical about its longitudinal diameter. Retardation in the growth 
3 - 
-- CL 
lu- 
ll 
Fig. 1. — Incomplete Development in Amblypneustes ovum. Apical area and surrounding 
coronal plates x 4^. 
of this area seems to have had the effect of preventing the apical system 
from attaining its customary polar position, so that, though still centrally 
placed ( i.e . immediately above the mouth), it is overtopped in height by 
the upper parts of radial areas I. and II. The apical system, but for the 
absence of the ocular plate corresponding to the incomplete ambulacrum, 
presents the normal pentagonal sjunmetry. A minor irregularity appears 
in inter-ambulacral area V., where, about 1 cm. from the edge of the peri- 
stome, there is a small papillary excrescence about 4 mm. in diameter. 
This is due to modification in three of the inter-ambulacral plates, two of 
which are occluded, and the intercalation of three additional small demi- 
plates which give rise to the protuberance. 
