PYGOPTERUS GREENOCKII, ETC. 705 



articulate in front with the pre-frontals, behind with the mastoids, internally 

 with the principal frontals, which they thus completely exclude from the orbital 

 margin. 



The mastoid (8) is very distinct, forming an elongated plate external to the 

 parietals, and articulating with them, and with the frontal and post-frontal bones. 

 Externally it is in contact with the facial bones ; and close beneath its anterior 

 part may be seen the attachment of the suspensory apparatus of the lower jaw. 

 Behind it forms, along with the parietal, the posterior margin of the cranial 

 buckler, and is succeeded in that direction by the suprascapular (50, fig. 2). 



Face. — Recurring to the profile view of the head, fig. 2, we observe that the 

 superior maxillary bone (21) is of great strength. Posteriorly it is very broad, 

 and overlaps the lower jaw near its articulation. Anteriorly it becomes, in the 

 orbital region, suddenly narrowed to a somewhat slender point, because of its 

 upper border being suddenly beveled off to form a curved margin, along which 

 the suborbital bone (73) articulates. The narrow extremity proceeds forwards 

 below the orbit to come in contact with the intermaxillary, and the external 

 surface is seen in fig. 1 to be ornamented with delicate striae, which run parallel 

 with the upper and hinder border of the bone. The posterior-inferior angle, over- 

 lapping the articulation of the lower jaw, is covered, however, with a minute 

 tuberculation. 



The intermaxillary (22) is a little bone bearing teeth, placed at the front of 

 the orbit, and beneath the snout and nasal openings. It is in connection above 

 with the pre-frontal ; internally and above with the nasal, behind with the 

 superior maxillary ; whether it articulates also with its fellow of the opposite 

 side is not seen distinctly. Externally it continues the orbital margin of the 

 cranial shield downwards and somewhat backwards, to which shield it is im- 

 movably articulated. 



The lower jaw (L, J) is stout and curved, the curve having the convexity 

 downwards. Of how many pieces it is composed it is impossible to determine, 

 though there are evidently at least two, viz. — dentary and articular. Its outer 

 surface is covered with a beautiful minute tuberculation, seen in fig. 1. 



Teeth may be observed on the superior maxillary and intermaxillary bones, and 

 also on the lower jaw. They are of two descriptions, large ones of about two 

 lines in length, alternating with smaller ones, of about half that size. These 

 teeth are sharply pointed, and of a slender conical form ; their external surface 

 is shining and smooth, their transverse section round, and the pulp cavity large. 

 Fig. 7 represents a portion of the edge of a lower jaw, with one large and several 

 smaller teeth attached. 



Opercular apparatus. — The operculum is an oblong, and somewhat rhomboidal 

 plate, its anterior, superior, and posterior inferior angles being rather acute. 

 Its upper margin is situated close beneath the suprascapular and mastoid bones, 

 and its long axis is directed at a very considerable angle downwards and back- 



VOL. XXIV. PART III. 9 F 



