Page:Quarterly Journal of the Geological Society of London, vol. 28.djvu/284

 the mandibular apparatus that of Cochliodus or Deltoptychus, while the premaxillaries are unlike those of any genus either recent or fossil. In proposing for it the generic title Prognathodus, I desire to express one of its most remarkable peculiarities — and in the specific appellation to recognize the obligation I am under to Dr. Gunther, not only for securing this valuable addition to ichthyology, but for his invariable kindness in assisting me with his unrivalled ichthyological knowledge on this and other occasions. The species alluded to as Ischyodus Johnsoni must be associated with Prognathodus in consequence of the identical characters of the premaxillary teeth ; but, as it differs specifically from P. Guntheri, it will retain its specific name.

EXPLANATION OF PLATE VIII.

Head of Prognathodus Guntheri, Egerton, nat. size : m, maxillary teeth ; p, premaxillary tooth; o, cranial plate?; 1, 2, 3, mandibular teeth.

Discussion. Dr. GUNTHER pointed out the interest attaching to the dentition of this fossil fish as being an additional evidence in favour of the connexion between the Ganoid and Chimaeroid forms. The existence of three teeth instead of one on each side of the jaw, as in Ceratodus and others, presented in it a generic character ; but the type was still the same. On one point he slightly differed from the view of the author ; and that was as to the application of the terms maxillary and premaxillary to the teeth. He thought the former belonged rather to the pterygo-palatine arch, and that the teeth in the front of the jaw should be regarded as vomerine. He illustrated this by reference to the jaws and dentition of Sharks, Chimaeroids, and certain Ganoids. In these the teeth, instead of being connected with the maxillary and premaxillary bones, were, in fact, connected with the pterygo- palatine arch. He considered that this furnished additional grounds for including all three forms in one subclass.

Mr. Etheridge made some observations as to the horizon in the Lias on which these fossil fishes occurred. He believed that nine out of ten of the Lower Lias species came out of the upper part of the Bucklandi limestone series. At the base of the cliff at Pinhay, Lyme Regis, are the Black Shales of the Rhaetic beds ; above them is the White Lias, in which there are no fish, though they occur on the same horizon elsewhere ; above these a series of shales with Ostrea, and above these again shales and limestones with Lima gigantea and Ammonites Bucklandi, the whole forming the Bucklandi series. The fish-beds (some 8 or 10 feet thick) contain about eighty species of fishes. Above this horizon fish are almost unknown in the Lias of Dorsetshire. At Barrow the fish also occur in the Bucklandi series, though somewhat lower down. In other eases fish- remains seem also to be restricted to certain horizons ; and the exact position of such remains as these was, in his opinion, an important feature in determining their distribution both in time and space.