OCR Text |
Show 1890.] UPPER CRETACEOUS FISHES. 633 deep series of the flank is characterized in the abdominal and anterior caudal region by a prominent mesial vertical ridge, nearly corresponding in position to the internal keel; and the chief ornament consists of irregular, thick, transverse rugae, which not only impart to the ridge a nodular appearance, but also frequently pass into a series of feeble crenulations at the hinder border. The dorsal and ventral scales are similarly rugose, but more finely marked ; and in the caudal region, too, there is a diminution in the prominence of the ornament. Generic Determination.-As already remarked, the species now described was briefly noticed by Agassiz under the name of Aspidorhynchus comptoni. In 1841, as at the present time, the jaws were undiscovered, and the most conspicuous character separating Aspido-rhynchus from Belonostomus was thus not available for reference. Two features now made known, however, appear to suffice for the generic determination of the fish with absolute certainty. The suborbital ring is in direct contact with the preoperculum throughout its length, there being no supplemental cheek-plate, such as characterizes Aspidorhynchus\; and only two series of flank-scales are deepened-one excessively so-while in Aspidorhynchus there are invariably three such series, more nearly equal in their vertical measurement2. These being special characters oi Belonostomus, and the Brazilian fish agreeing with the typical species of that genus both in the arrangement and proportions of the fins and in the development of the vertebral axis, there seems no reason to doubt the generic determination here adopted. Specific Determination.-The Brazilian Cretaceous fish is the largest species of Belonostomus of which any definite account has hitherto been published. The fragmentary skull from the Cretaceous of India, described under the name of Belonostomus (?) indicus3, will, if correctly determined, indicate even a slightly larger member of the genus ; but the smoothness of the external bones readily separates this form from the highly-ornate species now under consideration. The large English Cretaceous species i is also distinguished from the Brazilian fish, among other points, by the feeble character of its external ornamentation; and the small associated species5 is too imperfectly known for satisfactory comparison. The other Cretaceous members of the genus, B. crassirostris6 and B. lesind- 1 0. M. Eeis, " Ueber Belonostomus, Aspidorhynchus, und ihre Beziehungen zum lebenden Lepidosteus," SB. t. bay. Akad. Wiss., math.-naturw. Cl. 1887, p. 173, pi. ii. fig. 7. . . . 2 B. Vetter, " Die Fische aus dem lithographischen Schiefer im Dresdener Museum," Mitth. k. mineral.-geol. Mus. Dresden, pt. iv. 1881, p. 89. 3 Smith Woodward, "Description of a Fish-skull," Bee. Geol. Surv. India, vol. xxiii. (1890), p. 23. 4 Belonostomus ductus, L. Agassiz, Poiss. Foss. vol. n. pt. n. (1843), p. 142, pi. xvi. a. figs. 10-13; F. Dixon, Geol. Sussex, p. 367, pi. xxxv. figs. 3,3* ; Smith Woodward, Quart. Journ. Geol. Soc. vol. xliv. p. 145, pi. vii. figs. 7-13. s B attenuatus, F. Dixon, Geol. Sussex (1850), p. 368, pi. xxxv. figs. 4,4*. 6 O. G. Costa, Paleont. Eegno Napoli, pt. ii. (1856), p. 33, pi. ii. figs. 1, 2 (iucluding B. gracilis, Costa, ibid. p. 35, pi. ii. fig. 3). |