OCR Text |
Show 1890.] FROM THE WEALDEN AND PURBECK BEDS. 347 The head is of triangular shape, much laterally compressed, and measures about 0"18 in length by 0-16 in maximum depth at the occiput. The cranial roof is broadest at the occipital border, gently arched from side to side, and produced sufficiently downwards behind the eye to form a considerable portion of the posterior border of the orbital space ; there is a longitudinal median depression attaining its maximum depth and greatest breadth immediately behind the interorbital region and gradually becoming shallower in front and behind ; while none of the bones appear to have been ornamented, the only superficial markings being radiating rugae and ridges, evidently due to the ordinary processes of growth. No details of the cranial osteology can be deciphered ; but it is clear that there was originally no ossified interorbital septum, and there is a fragment of the parasphenoid (pas.) in position, which shows that this bone was narrow in the middle region of the skull. In the mandibular suspensorium, the hyomandibular (hm.) is conspicuous, but its inferior portion and connections are unfortunately destroyed ; the upper part of the bone is triangular in form, much expanded and laterally compressed, and at the median constriction the direction of compression becomes antero-posterior. Of the means of connection between this element and the quadrate, nothing can be discerned ; but the latter bone is well shown, in position, with its condyle adjoining the socket of the articular element of the mandible. The quadrate (qu.) is triangular in shape, thinning out at its notched superior border,-the binder margin being thickened, gently arched, turned outwards, and continued upwards as a long slender process ; while the articular condyle is somewhat constricted from the main part of the bone, and has a robust, inwardly-directed process arising from its base. The last-named process is only distinguishable on the left side of the fossil, but is there prominent and not readily explained. Of the pterygo-palatine arcade, three elements are seen from their outer lateral aspect. The metapterygoid (mpt.) is nearly as deep as long, triangular, with a thickened superior border and attenuated postero-lateral and antero-lateral margins ; it is gently bent, being outwardly concave behind and outwardly convex in front. The entopterygoid (enpt.) is broadest posteriorly, much elongated, with a nearly straight outer inferior border and an irregularly curved - inner superior border, these gradually converging to a rounded apex in front; the bony lamina is placed more nearly in a horizontal than in a vertical position, is sharply curved downwards at its posterolateral angle, and appears concave when viewed from the outer aspect above. The ectopterygoid (ecpt.) is a long, narrow, lamelli-form bone, apposed to the outer border of the entopterygoid, and apparently extending somewhat in advance of this ; but its exact form and proportions cannot be discerned. The premaxilla (pmx.) was evidently small, but only an imperfect fragment remains. The maxilla (mx.) is a long, narrow, laterally compressed, and gently arched bone, of almost uniform depth, except in its anterior third, which gradually contracts and ends in a stout, inwardly directed process ; the oral border is convex, and the hinder two-thirds of the |