OCR Text |
Show 1890.] FROM THE WEALDEN AND PURBECK BEDS. 349 rakers (the fragmentary rods of the type specimen), each of these being smooth and elongated, with a slight constriction immediately above its base, and tapering to a point distally. Vertebral Column.-A single centrum attached to the occipital portion of the type specimen (Plate XXVIII. fig. 1 a) shows that the vertebrae were completely ossified, while the second Wealden fossil and some of the Purbeckian specimens reveal the principal characters of the anterior part of the vertebral column. The centra are narrow and distinctly amphiccelous, much deeper than broad, and marked on the sides by fine transverse striations extending between a thickened rim anteriorly and posteriorly ; a pair of deep pits on the upper aspect accommodates the neural arch, and there is a similar pair of pits on the ventral aspect for the insertion of a haemal arch. The only traces of attached peripheral elements on the sides of the centra consist in a small, faint, rounded pit or rugosity on four or five of the anterior vertebrae in the so-called Lepidotus, which may have supported an intermuscular bone. The first vertebral centrum, articulating with the basioccipital, is composed of two thin discs fused together (Plate XXVIII. fig. 3), but the others are all simple, each bearing its own arch. The neural spines (Plate XXVIII. fig. 4) are long and slender, fixed to delicate, low arches, with prominent zygapjphyses; and if the fossil that best displays these structures gives equally reliable indications of the haemal elements, the latter have the form of very feebly developed ribs. Two long curved bones in the second of the large Wealden specimens are also at first sight suggestive of ribs, and seem to indicate a greater development of these structures than is here shown ; but the elements in question are not certainly determinable and may be branchiostegal rays. Generic and Specific Determination.-That the fossils now described pertain to the genus Oligopleurus seems evident from the form and proportions ot the jaws and dentition, the characters of the vertebral centra, and the slight development of the neural and haemal arches. The absence of scales is explained by their extreme tenuity in the fish of the Lithographic Stone, and the coarse nature of the matrix in which the new Wealden and Purbeck specimens occur. These fossils, however, scarcely suffice for a specific determination, and unless the small immature individual from the Purbeck beds, shown, of slightly reduced size, in Plate X X I X . fig. 3, be the young of the form under consideration, no precise diagnosis can as yet be attempted. That the larger fossils now described represent a distinct species from the typical 0. esocinus, seems to be indicated by the narrowness and greater relative depth of the anterior vertebrae in the English specimens; and they may thus be provisionally named O. vectensis, from the discovery of the first and best-preserved fossil in the Isle of Wight. If, however, the small fish just referred to prove to be truly referable to the same species, it will readily be distinguished by its much more slender proportions-the depth of the trunk at the position of the pectoral arch being comprised nearly seven times in the total length, whereas in 0. esocinus the same |