1847.] EGERTON ON THE FOSSIL CHIM^ROID FISHES. 351 



and Goldworth Hill specimens, I propose to place them under Buck- 

 land's genus Edaphodon, and to cancel the more recent name Psitta- 

 codon. The validity of this arrangement will be apparent by the 

 following distinctive characters of the two genera. The lower max- 

 illary oi Edaphodon (PI. XIII. fig. 3) differs from the corresponding 

 part of Ischi/odus (PI. XIII. fig. 4) in being produced anteriorly into 

 a falciform beak. The symphysial facet (fig. 3 a) is broad at the 

 base, and contracts gradually forwards until the margins meet at the 

 apex. In Ischyodus the lower jaw is deeper, less produced anteriorly, 

 and the margins of the symphysis (fig. 4 «) are parallel until abruptly 

 truncated at their extremities. The upper maxillary of Ischyodus 

 (PI. XIII. fig. 1) is easily distinguished by the occurrence of a fourth 

 triturating surface (PI. XIII. fig. 1 e), situated between the large tuber- 

 cle b and the marginal one d. This is wholly wanting in the corre- 

 sponding bone of ^c?«^Aoc?o^ (PI. XIII. fig. 2). The premaxillaries 

 are comparatively rare in the fossil state, only three species having 

 been found since the original Shotover specimens which first led 

 Dr. Buckland to the discovery of the true affinities of these enig- 

 matical fossils. I have assigned these forms to Ischyodus Townshendi, 

 Edaphodon Mantelli and Edaphodon gig as, characterizmg the two 

 latter as being (compared with the former) broader, more com- 

 pressed and less robust in antero-posterior diameter, and more hooked 

 at their extremities. On looking over Mr. Bowerbank's collection 

 of Bracklesham fossils not long since, I was led to imagine that 

 the genus Passalodon might prove to be the premaxillary appara- 

 tus of Edaphodon. In favour of this idea the following reasons sug- 

 gested themselves. The two genera have in every case been found 

 associated together in the same strata, yet although both the upper 

 and lower maxillaries of several species of Edaphodon have been dis- 

 covered, the premaxillary has been hitherto a desideratum, while only 

 one form of Passalodon has ever been brought to light. This form 

 has a striking resemblance to the premaxillaries of the two species of 

 Chimseroids now transferred to the genus Edaphodon. As to structure, 

 the arrangement of the dental substance in distinct columns separated 

 by septa of bone, is precisely that which occurs in the corresponding 

 parts of the other fossils of this family. Upon communicating these 

 views to Mr. Dixon, he wrote to me as follows : " I can bring forward 

 very strong geological evidence in support of your opinion that Pas- 

 salodon is part of Edaphodon, for I have found them together. The 

 double upper jaw of Edaphodon which I possess, had portions of 

 Passalodon with it.'* Since the receipt of this letter, Mr. Dixon has 

 forwarded me a specimen of Passalodon which fully establishes the 

 fact, for at the posterior extremity of the bone a flat articulating 

 surface is preserved, corresponding accurately with the anterior arti- 

 culating surface of the upper maxillary bone of Eda'phodon. 



Since my former communications on the Chimaeroid fishes, the 

 anatomical details of the dental apparatus of Elasmodus have been 

 completed by the discovery at Sheppy and Bracklesham of the upper 

 maxillaries and premaxillaries of Elasmodus Hunteri. The remark- 

 able laminated structure which suggested the generic name, obtains 



