616 MR. J. W. DAVIS ON" CHONDKOSTETTS ACIPENSEROIDES. 



the thicker ossification appears to depend more on the age of 

 the fish than anything else. The evidence of the anal fin is 

 derived from very fragmentary specimens. The anal fin of C. 

 crassior is described as more massive and having the transverse 

 articulations at shorter intervals than the anal fin of C. acipen- 

 seroides — the latter being represented in the specimen depicted .on 

 plate lxvii. (Phil. Trans.), and the former, in conjunction with the 

 caudal fin regarded by the author as belonging to the species 

 C. crassior, on pi. lxx. It was unfortunate that no specimen 

 was known to Sir Philip Egerton having the body sufficiently 

 well preserved to show the actual relationship of the caudal fin 

 with the remaining constituents. The specimen herein described 

 proves that there is no difference between the tail described as 

 belonging to C. crassior and the one actually attached in this 

 specimen to the body of C. acipenseroides, so that the former must- 

 be united with the latter species. It is very probable that better 

 preserved specimens will exhibit an equal similarity in the anal fins ; 

 and until such are found it must be remembered that the anal 

 fin on which Sir Philip Egerton based his determination is the 

 one attached to the caudal fin mentioned above, and which is 

 undoubtedly the tail of C. acipenseroides, Ag. 



EXPLANATION OF PLATE XXIII. 



Specimen of Chondrosteus acipenseroides, Ag., from the Lias of Lyme Regis, 

 one-sixth natural size, op, operculum ; p, pubis (?) ; v, ventral fin. 



Discussion. 



The President observed that the subject was one to which the 

 Author had devoted much labour, and that the comparison of such 

 fossil forms with existing ones was of the greatest importance. 



Mr. Smith Woodward regretted that there was no specimen to 

 verify the drawings, which seemed to show the bones with unusual 

 clearness. A figure of so instructive a fossil would be especially 

 valuable : but he advocated a more modern nomenclature than that 

 of the Author. He considered the absence of an anal fin accidental. 

 These Chondrostei of the Lias formed the connecting-link between 

 the old PaloeoniscidaB and the living Sturgeons, notably Polyodon, 

 and any contribution to their anatomy was thus very welcome to 

 ichthyologists. 



Mr. Newton remarked on the fact that the tail alone was all that 

 Agassiz had to work upon. In 1858 Egerton gave further descrip- 

 tions, including parts of the head, but his specimens were crushed. 

 Mr. Davis's specimen was more perfect, and for the most part the 

 bones of the head appeared to be in the position they ought to 

 occupy. He criticized the position of the coracoid with reference 

 to the scapula, the line between them being, as he thought, due 

 to a break in the stone. The appearance of the mouth might 

 also be due to breakage. He referred to the absence of the anal fin 

 and to the apparently disjointed condition of the vertebral column. 



The Secretary observed that the Author expressly noted the 

 absence of the anal fin. 



