3.30 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [Jan. 31, 



does not extend. No incisor teeth are visible, but it is probable, 

 from the prominence of the anterior angle of the jaws, that they were 

 furnished with teeth of more elongated form than those constituting 

 the masticatory apparatus. 



Having thus obtained a clue to the natural affinities of this genus, 

 I was agreeably surprised to find, on a close comparison with the other 

 members of the Pycnodont family, how closely they are allied both in 

 form and structural details. Unwilling, however, to rely entirely on 

 my own judgement in this matter, I communicated the facts to Pro- 

 fessor Agassiz, and received from him the following corroboration of 

 my views which I give in his own words : — '* I quite agree with you 

 in the propriety of combining the genus Platysomiis with the Pyc- 

 nodonts ; for some time past I had, indeed, been impressed with the 

 great difference there is between that genus and the others of the 

 family in which it stands, and I now feel that my only reason for 

 putting it there was the heterocerqual form of tail, a character which 

 could not fail to produce a vivid impression upon my mind when first 

 discovered, but which I now expect to find in fishes of various fami- 

 lies in the oldest geological ages, as well as everywhere in the youngest 

 state of our actual fishes in their embryonic growth. The teeth, as 

 you mention, are conclusive evidence for placing Platysomus with the 

 Pycnodonts. Let me now point out to you another evidence of this 

 relation in the form of the skeleton, especially of the apophyses before 

 the dorsal. The specimens of Platysomus in the museum in Munich 

 show some good portions of the skeleton, and in my mind I can now 

 compare them to the skeleton of the small Pycnodus rhombus *, 

 without detecting any difference. Pray institute the comparison upon 

 a safer ground than recollection, and let me know what you find. 

 You know under what circumstances the fossil fishes have been 

 worked out, and as a matter of course I must expect to see daily 

 important additions made to the edifice of which I have laid only the 

 foundation." 



I had not neglected the important comparison above alluded to, 

 and the result proved that the correspondence between the anatomical 

 details of Platysomus, Gyrodus and Microdon is very remarkable. 

 The peculiar features described by Agassiz as " apophyses before the 

 dorsal " obtain in Platysomus as in the other Pycnodonts, but whereas 

 in the genus Pycnodus they are restricted to the area in advance of 

 the dorsal fin, in Platysomus and Gyrodus they extend over the hinder 

 portion of the body. As to the nature of these so-called apophyses, 

 I have arrived at a conclusion at variance with that advocated by 

 Agassiz, and consider them as belonging to the tegumeutary invest- 

 ment of the fish, rather than to the internal skeleton. It is scarcely 

 necessary to repeat the minute and accurate description of these bones, 

 given in the article in the ' Poissons Fossiles' on the genus Pycnodus^ 

 as they must be familiar to every student of this branch of Palaeon- 

 tology f. The conclusion Agassiz draws is, that he considers them as 

 the analogues of the V-shaped bones in the Clupeidce. My reasons 

 for differing from this great authority are these : — First, they are 



* Poiss. Foss. vol. ii. tab. 72. f. 5-7. t Ibid. vol. ii. pt. 2. p. 183. 



