OCR Text |
Show 1890.] FROM THE WEALDEN AND PURBECK BEDS. 351 the support of the hsemal arches. There are also long slender ribs and neural spines. Appendicular Skeleton.-The basal portion of the pectoral fin (p.) consists of stout unarticulated rays, not less than fifteen in number ; but the distal part of the appendage is unfortunately wanting. Scales.-There is distinct evidence of a well-developed squamation. The scales are thin, ganoid, and smooth externally, and appear as if deeply overlapping. Generic and Specific Determination.-The principal characters of the fossil thus described show that it pertains either to Caturus or Strobilodus; and, as remarked by von Zittel, the known differences between these two types are so slight that it is probable the latter must only be regarded as a subgenus of the former. Since, however, the obliquity of the mandibular suspeusorium is greater and the ossification of the vertebrae apparently more advanced than in the typical species of Caturus, while the teeth exhibit bases as tumid and as much indented externally as those of the typical Strobilodus, it seems advisable to adopt the latter name for the Purbeckian fossil, whatever its value may eventually prove to be. Of this genus, S. gigas, from the Lithographic Stone of Bavaria1, and S. suchoides, from the Kimmeridge Clay of Norfolk 2, are the only two recognized species ; and the new fossil is distinguished from both by attaining scarcely half the size, by the relative narrowness of the maxilla and mandible, the slenderness of the apical portion of the principal teeth, and the extreme elongation of the hindermost maxillary teeth. It may thus be regarded as indicating a hitherto unrecognized species, to be named S.purbeckensis, from its occurrence in the Purbeck beds, which constitute the highest horizon as yet known to yield remains of the genus in question. Genus MESODON. [A. Wagner, Abh.k.-bay. Akad. Wiss., Cl. ii. Bd. vi. 1851, p. 56.] MESODON DAVIESI, sp. nov. (Plate XXVIII. fig. 5.) A typical member of the Pycnodont genus Mesodon, from the Purbeck beds of Swanage, is indicated not only by the well-preserved fish shown in Plate XXVIII. fig. 5, but also by the head and anterior abdominal region of another individual, which may possibly pertain to a distinct species. The total length of the complete specimen is 0*225, the figure being thus one half of the natural size. The fish is round and short, the maximum depth of the trunk immediately in advance of the dorsal fin being somewhat less than the length of the head and trunk to the base of the caudal fin ; while the head and opercular apparatus are contained nearly five times in the total length (including the caudal fin). Head and Opercular Apparatus.-The bones of the head are much crushed and obscurely defined, the majority being partly broken away, and some only displaying the fibrous inner aspect. The skull 1 A. Wagner, torn. cit. p. 75, pi. ii. 2 Thlattodus suchoides, R. Owen, Geol. Mag. vol. iii. (1866), p. 55, pi. iii. |