236 PKocEKDiNGs OP THE GEOLOGICAL sociEri'. [Marcli 6, 



the mandibular apparatus that of Cocliliodus or DeltoptycMus, while 

 the premaxillaries are unlike those of any genus either recent or 

 fossil. In proposing for it the generic title Fror/natJiodas, I desire 

 to express one of its most remarkable peculiarities — and in the 

 specific aj)pellation to recognize the obligation I am under to 

 Dr. Giinther, not only for securing this valuable addition to ichthy- 

 ology, but for his invariable kindness in assisting me with his 

 unrivalled ichtliyological knowledge on this and other occasions. 



The species alluded to as Ischi/odus JoJinsoni must be associated 

 with Prognathodus in consequence of the identical characters of the 

 premaxiUary teeth ; but, as it differs specifically from P. Gilntlieri, it 

 will retain its specific name. 



EXPLANATION OF PLATE VIII. 



Head of Prognathodus Gimtheri, Egerton, iiat. size : m, maxillary teeth ; 

 ^, premaxiUary tooth; o, cranial plate?; 1, 2, 3, mandibular teeth. 



Discussion. 



Dr. GtTNTHER pointed out the interest attaching to the dentition of 

 this fossil fish as being an additional evidence in favour of the con- 

 nexion between the Ganoid and Chimoeroid 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 diff'ered from the view of the author ; 

 and that was as to the application of the terms maxillary and premax- 

 iUary 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, ChiniDeroids, and certain Ganoids. 

 In these the teeth, instead of being connected with the maxillary 

 and premaxiUary 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. Ethertdge made some observations as to the horizon in the 

 Lias on wdiich these fossil fishes occurred. He believed that nine out 

 of ten of the Lower Lias species came out of the uj^per part of the 

 Buclclandi limestone series. At the base of the cliff" at Pinhay, 

 Lyme Regis, are the Black Shales of the Rhsetic 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 audi Ammonites Bucldandi, the whole forming the BucHandi 

 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 BiicTc- 

 landi series, though somewhat lower down. In other eases fish- 

 remains seem also to be restricted to certain horizons ; and the exact 

 position of srich remains as these was, in his opinion, an important 

 feature in determining their distribution both in time and space. 



