104 H, L. Hawkins—On Lovenia forbes. 
oculars I and V to the ambitus in area 5 is greater than the corre- 
sponding measure in any other area, and here we find but a bare 
indication of compression in a small reduction in the height of a few 
plates. A table of these measurements in the two specimens described 
will emphasize this point. 
Number of plates in each iamb. column, 13. Distance (in mm.) from edge of 
ocular plates to ambitus— 
Radius II. Interradius 1. Interradius 5. 
ALA i 15-9 16-2 19-8 
Bist ‘ 16-0 16-2 19-0 
Average height (in mm.) of one iamb. plate in these positions— 
AME tee : 1-223 C 1-246 C 1-523 
B } : 1-231 C 1-246 C 1-461 
(C indicates development of plate-crushing.) 
In specimen B, where the distances are relatively shorter, but the 
normal plates actually larger, than in specimen A, the degree of 
plate compression is greater. In view of these conditions, it seems. 
impossible to resist the conclusion that simple mechanical pressure has 
been the prime cause of this unusual development. The growth of 
tubercles (to which Lambert would ascribe much influence in 
ambulacral plate-crushing) can have no effect in this case, for the 
scattered tubercles, though large for a Spatangid, are far removed 
from the margins of the plates and have ample room for extension. 
In comparing the development of plate-crushing in this aberrant 
case with the similar structures in the ambulacra of many other 
Kchinoids, the place at which the reduced plates occur becomes an 
important feature. It is demonstrably at the line of junction between 
the unchangeable and immovable adoral plates and the less specialized 
movable plates higher in the column; that is, the latter plates are 
crushed against the barrier. 
In the case of the ambulacra (and interambulacra) of the Cidaroids. 
there is nowhere any hindrance to the progress of the plates from 
apex to peristome, and no plate-crushing appears. In the other 
Regular Echinoids, and in the Holectypoida, the major part of the 
perignathic girdle is established on the ambulacral plates next to the 
peristome, and must, to exercise its function, remain there constantly 
and unchanged. At once plate-crushing appears (in the ambulacra), 
at the peristome margin first, extending farther up the area as 
specialization progresses. In Lehinocardium cordatum, as I have 
shown recently, an abnormal rate of plate-growth occurs in ambu- 
-lacrum III; but those plates of the area that are on or below the 
ambitus have already taken on the large, immobile character of the 
plates of a Spatangid adoral surface. The new plates thus crush 
against a barrier situated on the adapical surface, and the compound 
plates are restricted to the ‘petaloid’ region. In the closed petals of 
Clypeaster, Echinarachnius, and Laganum, a similar block occurs at 
the outer limit of the petal, and crushed plates make their appearance 
within the petals of fully-grown specimens. 
The case of Zovenia under notice is a further, and even more 
obvious, illustration of the theory of plate-crushing in Kchinoids. 
