OCR Text |
Show 630 MR. A. SMITH WOODWARD ON SOME [Nov. 18, 1844. Aspidorhynchus comptoni, L. Agassiz, Comptes Rendus, vol. xviii. p. 1009. The horizon from which the Brazilian fossils were obtained has long been well known. The fish-fauna was briefly noticed by Agassiz so long ago as 1841 and 1844 (loc. cit.), and Prof. Cope described one of the genera in 1871 \ while the present writer treated another genus at still greater length in the 'Proceedings' of this Society, June 23rd, 1887 2. As in the case of Rhacolepis, already described, the examples oi Belonostomus occur in nodules in a beautiful state of preservation, though, on account of the form of the fish, the specimens are always incomplete. As a rule, the long body is bent upon itself at about the middle point, the tail thus lying in close proximity to the head (Plate LIV.) ; and in no instance is the slender elongated snout completely preserved. Several typical portions of the fish are shown in the accompanying drawings (Plates LIV., LV.), and the Brit. Mus. register-numbers of some of the more important specimens are placed in brackets after the various descriptions of anatomical characters which they specially demonstrate. All measurements are given in decimal fractions of the metre. General Form.-Owing to the death-contortion, it is not readily possible to estimate the precise proportions of the fish under consideration. The trunk, however, must have sometimes attained a total length of not less than 0\55 ; and the maximum depth of such an individual, shortly behind the pectoral arch, would be about 0-08. The total length of the head and opercular apparatus of a fish of this size would probably not exceed 0'24. As usual in the genus, the head and trunk are much laterally compressed, and the fins are relatively small. Head and Opercular Apparatus.-The long, narrow cranial roof is flattened in the middle and beautifully ornamented with close, thick, vermiculating rugae of ganoine, which have numerous short branches, and are chiefly disposed in a longitudinal direction upon the rostral region (no. 15495 a). Behind the parietals, a pair of large supra-temporal plates continues the roof backwards as far as the hinder extremity of the upper border of the operculum (no. P. 975 6). In advance of the frontals, the snout tapers rapidly into a very slender rostrum, of which the base is shown from above in Plate LV. fig. 1. Seen in profile (no. P. 3810), the much elongated frontal and rostral region inclines gradually downwards from the short parietal region, which continues the dorsal plane of the trunk ; and the narrow, well-developed parasphenoid bone is parallel with the parietal roof. There are extensive ossifications in the otic region, but no interorbital septum occurs. A remarkable pair of large longitudinal tubular ossifications is also shown in transverse sections of the rostrum (Plate LV. fig. 2), these structures extending almost or quite as far backwards as the orbital space. They are probably ethmoidal in character, and destined for the protection of the elongate pedicles of the olfactory lobes. The bones of the mandibular suspensorium 1 Ancedopogon, E. D. Cope, Proc. Amer. Phil. Soc. vol. xii. (1871), p. 53. (Founded upon the undescribed Cladocyclus garclneri, Agass.) 2 Rhacolepis, Smith Woodward, P. Z. S. 1887, pp. 535-542, pis. xlvi., xlvii. |