8 ON THE STRUCTURE OF TRACHYPTERUS ARCTICUS, 
The Suspensorium and the Jaws (Plate I.). 
The hyomandibular is very long and rod-shaped, presenting a broad end above, 
capped by cartilage, for articulation with the region of the auditory capsule ; below 
it is pointed, and articulates with the inter-hyal. The bone, as a whole, presents an 
anterior and an internal face, and posteriorly a groove formed by their sides, which 
is replaced near the head of the bone by a rounded surface merging into the inner 
side. Below this rounded surface, and just internal to the top of the groove, the 
operculum is definitely articulated. More than the upper half of the anterior edge of 
the preoperculum is lodged in the groove. Between the anterior and inner faces is a 
sharp angle, which below comes into relation with the metapterygoid. The pointed 
inferior extremity is articulated to the nodule of cartilage which caps the inter-hyal, 
and this nodule separates it from the symplectic. It gives support to the pseudo- 
branch or opercular gill (P/. I., Fig. 2). 
The quadrate is a triangular flattened bone. At the lower corner it is trans- 
versely thickened to form a horizontal condyle for the articular: the lower border is 
a flat one, enclosing a groove on the inner side of the bone for the lodgment of the 
symplectic. The anterior edge underlies the pterygoid. The upper border articulates 
with the mesopterygoid anteriorly: in this region, between the quadrate, meso- 
pterygoid, and metapterygoid, are copious remains, as PARKER has pointed out in 
Regalecus, of the embryonic palato-pterygoid cartilage. 
The symplectic reposes in a hollow on the inner side of the quadrate, beyond the 
posterior border of which it only slightly projects. It is a narrow bone, tapering as 
it passes into the quadrate. 
The pterygoid is a narrow, flat bone, with pointed lower extremity. It 
articulates by its upper edge with the palatine, and by its posterior edge with the 
mesopterygoid and the quadrate. 
The mesopterygoid is the largest of the series. It is irregularly four-sided in 
outline. Its outer surface is divided into an upper and lower half by a ridge-lke 
horizontal: fold. It articulates in front with the palatine and pterygoid, below with 
the quadrate, and posteriorly with the metapterygoid. 
The metapterygoid is an excessively thin element, of a triangular shape. It 
articulates with the mesopterygoid, and below comes into relation with the common 
cartilage, uniting the quadrate and symplectic to the mesopterygoid and inter-hyal. 
The palatine is a flat, rounded bone, with a well-developed head for articulation 
with the parethmoid and the pre-nasal cartilage. Below it is closely attached by 
membrane to the pterygoid, and it overlaps the mesopterygoid posteriorly. 
The articular is a very large triangular bone, fitting into the dentary anteriorly, 
and forming about one-half the skeleton of the mandible. The upper border is 
straight and thickened, and presents posteriorly a large concavity for the articular 
