292 Proceedings of the Royal Society of Edinburgh. [Sess. 
hinge movement, the necessity for which we can appreciate when we con- 
sider how short are the attached muscular fibres and how greatly excentric 
is the axis of the mandibular body as compared with that of the mandibular 
joint. On the medial aspect, where it comes in contact with the pharyngeal 
process of the alar plate, the adductor calcification is smooth and flattened. 
The two adductor muscles take up a large part of the total space within 
the cephalon. Each is shaped like an inverted pyramid, and takes origin 
from almost the whole sculptured region of its own half of the head. 
At their origin the right and the left muscle meet in the middle line, and 
between them the foregut rises to a sharp ridge like the roof of a house 
— see fig. 21. Each abductor muscle, taking origin lateral to the corre- 
sponding adductor, is slender, and acts with slight mechanical advantage, 
the abductor process extending to a less distance from the axis of the joint 
than does the attachment of the adductor calcification. The mandible is 
capable of an angular movement of nearly 40 degrees. 
Within each mandible is left a cavity of considerable size, which is 
occupied with glandular tissue. Neither palp nor lacinia mobilis are 
present on the external aspect. In complete adduction the curved incisor 
edge of the right mandible is received within the concavity of the corre- 
spondingly curved incisor edge of the left, while to prevent slip an 
additional isolated incisor process on the left mandible is received within 
the concavity of the curved incisor edge of the right. 
The Maxillce . — The only figures showing the complete skeleton of the 
maxillae in any of the Yalvifera are to be found in a paper by Hansen 
(1886).* The skeleton of the distal parts of the maxillae of Glyptonotus 
has already been figured ; that of the more proximal parts is less clearly 
defined than would appear to be the case in Chiridotea. 
The basal segment of the first maxilla consists of a solid chitinous or 
but slightly calcified rod (it is flexible), which articulates with the inferior 
border of the alar plate, a rounded cavity in the border being prepared 
to receive the head of the segment. Up to the point of attachment of the 
medial lobe this proximal rod is stout ; beyond this point it thins down, no 
second element being applied to it as in the case of Chiridotea. 
The two lobes spring from the proximal rod at a right angle, the 
skeleton of the medial lobe being in its proximal half a mere localised 
thickening of the surrounding articular membrane, the corresponding part 
of the lateral lobe being a calcified gutter open on the medial aspect. 
While the freely projecting distal half of each lobe is hollow, the proximal 
* These figures, which refer to the maxillae of Chiridotea , have been copied in Caiman’s 
book (1909). 
