486 : MR. P. GEDDES ON THE MECHANISM 
closely fitted by its non-dentigerous flaps. In action the powerful internal retractor 
muscle (d/m), aided by the external retractor (ed/m), must drag slightly upon the apex 
of the radula, but mainly upon the apex of the infraradular sheet ; and both, prevented 
from sliding over the apical edge of the framework by their sharp permanent curvature 
in both longitudinal and transverse sections, as well as by the parallel curvature and 
absence of firmness in the framework itself, must be pushed upwards and forwards, the 
teeth of the radula thus scraping harshly against an object held in the jaws immediately 
above. During this upward and forward curvature, if the dorsal transverse fibres of 
the framework (dtf) contract, they will assist the stroke, at the same time deepening 
and narrowing its cavity. The return is effected by the elasticity of the whole 
apparatus, assisted by the contraction of the ventral longitudinal muscles of the infra- 
radular sheet (v/s, figs. 1, 4, 5), as also by the ventral transverse fibres of the support 
(rtf, figs. 1, 4, 8). A pretty good rough idea of the action may be gained by laying 
the open hand on the table with the palm upwards and bringing up the four finger- 
tips till they nearly meet, the knuckles being meanwhile pushed forward along the table, 
and then drawn backwards as the hand reopens—or even by moving the tip of the 
tongue, as in licking, curling up its sides at the same time, to represent the sulcus. 
Attention is particularly called to the fact that the construction and adjustment of 
each and all of the three parts of the whole mechanism—the radula, the infraradular 
sheet with its muscles, and the framework, in all the details above described—is such as 
to render a strap-over-pulley motion practically impossible. 
In Patella (figs. 10-25) the framework is very highly developed, containing three 
pairs of cartilages. The anterior pair is continuous for a short distance in front, and 
the pointed apex, though slightly rolled downwards, is very thin and flexible. The 
ventral pair of protractor muscles, united into a continuous sheet (pr, fig. 14), originate 
in the lower lip ; the postero-lateral (/pr, fig. 20) at the sides of the head. Both pairs 
are inserted into the posterior buccal cartilages; but while the lateral muscles are attached 
at about the middle of these, so as to pull the whole buccal mass forward without 
moving its parts upon each other (as also do the anterolateral pair), the lower sheet is 
attached to the ventral edge of the cartilages (figs. 14, 20); and these, therefore, rock 
upon the anterior pair at the joint, thrusting them upwards and forwards. 
The infraradular sheet fits over the anterior pair of cartilages, but is too thin to have 
a permanent angle at the apex as in Loligo. Its ventral muscles originate in the 
posterior buccal cartilages’; and their contraction must push the anterior pair, and thus 
the apex, downwards. The powerful muscles which antagonize them on the dorsal 
side must bend the cartilages upwards at the apex through a not inconsiderable arc, 
the sulcus at the same time deepening. It will be seen, however, that in this case the 
‘ These cartilages seem to be represented in oligo by a membranous sheet similarly placed, and from which 
the dorsal and ventral muscles of the infraradular sheet originate. 
