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Show 560 MR. A. S M I T H W O O D W A R D O N T H E [June 20, essential features in the osteology of the group to which it belongs. The exoskeleton is so robust that it is usually well preserved, and the internal bones also exhibit a high degree of ossification. As found in the Oxford Clay of Northamptonshire and in the Wealden of Sussex, the remains are in an especially good condition ; and the discoveries of Messrs. Alfred X . Leeds and Samuel II. Beckles respectively render it possible to obtain information concerning nearly all the more important characters of the skeleton. In the case of one Oxfordian specimen from the Leeds collection in the British Museum (no. P. 6841 *), all the bones are isolated as if carefully macerated; in others the display of the various elements depends upon accidents in fracture. It is the object of the present communication to describe these specimens so far as they add to our knowledge of the cranial osteology of Lepidotus; and finally to compare with the skull of this genus that of the closely related Liassic fish, Bapedius. I. O N T H E C R A N I A L , FACIAL, A N D O P E R C U L A R B O N E S OF Lepidotus. The basicranial axis in Lepidotus is straight, and the cranium itself is well ossified. The basioccipital element, known only from a laterally compressed specimen (Plate X L I X . figs. 1 a, 1 b), is highest behind and exhibits on its posterior face a very deep conical fossa for the notochord (n.f.). Towards the postero-inferior angle of each side there is a small acuminate tubercle or process (x); and the inferior aspect of the bone exhibits a broad longitudiual groove, with a flattened rim on each margin, evidently indicating the backward extension of the basicranial canal (for the recti muscles of the eye) as far as the occiput. There is some appearance also of one vertebral ring (v.) being fused with the basioccipital on its hinder face. The exoccipital (fig. 1) forms a considerable part of the lateral wall of the brain-case, and is perforated as usual by a large, transversely oval foramen (/.) for the exit of the vagus nerve. The pro-otic (fig. 2) has a still larger extent than the exoccipital, and exhibits a deep, oblique notch (/.) at its anterior margin for the passage of the fifth nerve. In the interorbital septum there is as yet no evidence of ossification, and particulars are similarly wanting as to the nature of the pterotic and opisthotic bones. Of the rostral region it can only be said that the ethmoids are arranged in such a manner that the olfactory nerves must have passed through a pair of long, closed canals, well shown in a Wealden fossil in tbe British Museum (Plate X L I X . fig. 4, etli.). 1 This is the type specimen of a new species, Lepidotus latifrons, diagnosed in the forthcoming Oatal. Foss. Fishes, Brit. Mus. pt. iii. Its principal distinctive characters are:-External head- and opercular bones ornamented with conspicuous tuberculations; all the teeth with much elevated crowns. Premaxilla with six teeth. Each frontal bone twice as long as its maximum breadth ; operculum three times as deep as broad. |
