OCR Text |
Show 534 MR. A. S. WOODWARD ON [Dec. 7, and for two thirds of its extent it is directed outwards and exhibits only very slight tapering; the distal third rapidly diminishes to a pointed extremity and is fixed at right angles to the rest in a backward direction. The hinder appendage (ci.b) measures only two thirds the length of the first, and is about half as broad ; it likewise has an outward and posterior trend, but (in its fossilized state) is much more gradually arched. The relatively great size of' these appendages renders them somewhat difficult of interpretation, and it is scarcely possible to decide whether they consist of true cartilage or are merely dermal in character. In position they correspond very closely with the hinder labials of many living Selachians, and also with the supposed homologous rods that form the axes of the oral barbels in Myxinoids1. But in the latter group these appended " feelers " never seem to extend outwards to a length much exceeding half the breadth of the head ; and the largest cirri with which I a m acquainted in the Selachians are scarcely longer than the rami of the jaws. It seems likely, however, that the structures in Squaloraja are the gigantic representatives of the latter, which are elongated outgrowths of the cartilages of the nasal valves2. There is no evidence of anterior labials in the specimens of Squaloraja already known. Dentition.-The dentition of Squaloraja is very remarkable and has not hitherto been correctly noted. Some indications are to be observed in specimens I. and II., but the teeth are beautifully displayed both in III. and IV.; moreover, the parts in these two instances are practically identical, showing that there were no variations according to sex, as is the case among certain living Selachians (e.g. Raja). The dentition of both jaws is preserved in no. III. (figs. 3, 4), but only that of the right side of the mandible affords a good view of the grinding-surface. Each ramus bears only a single dental plate, sharply marked off in front from its fellow of the opposite side, and exhibiting towards the symphysis a gently tumid prominence. For the anterior two thirds of its length the plate is of nearly uniform breadth, but in the last third the outer border gradually trends inwards, producing a more or less pointed posterior extremity. And the efficiency of the grinding-surface is increased by a series of parallel longitudinal ridges or folds of the enamel, which are distinctly worn down towards the outer functional border. Eleven of these rugae can be counted on no. III. (fig. 4 ) , while fourteen or fifteen are visible in no. IV ; but, except on the under surface of the plate in no. IL, there are no traces of any sutures between them, and even in this specimen the evidence is somewhat obscure. It appears, however, that there were feebly marked longitudinal divisions corresponding to the several rugae, and that these became accentuated at the outer edge, allowing of the shedding of the worn-out portions as growth proceeded. 1 W . K. Parker, loc. cit. pp. 385, 399, pis. x., xvii. figs. 1-3. 2 The " Nasenfliigelknorpel " of Miiller ; see 0. Gegenbaur, ' Das Kopfskelet der Selactner,' pp. 97-111, pis. xvi., xvii. |