LINNEAN SOCIETY OF LONDOX, 3! 



The discussion upon the Origin oE the Vertebrates, begun at 

 the previous Meeting, was resumed. 



The discussion was continued by Dr. A. Smith Woodward, 

 r.E.S., F.L.S., who remarked that Paheontology affords no clue 

 to the ancestry of the Vertebrates, because they seem to have 

 originated as animals with no hard parts caj)able of fossilization. 

 When they first acquired a calcified skeleton in the Upper 

 Silurian period, they were represeiited not only by very primitive 

 types like the Ostracoderms, but by true fishes of at least as high 

 a grade as the Elasmobranchs (Acanthodians). 



It is perhaps a significant fact that the Arthropods were the 

 dominant type of life at the time when the Vertebrates began to 

 be conspicuous. It is known that during the subsequent course 

 of evolution of the Vertebrates themselves, each successively 

 higher great group became the dominant type for the time being ; 

 and that each advance was due to evolution from the immediately 

 precednig dominant type. In every case, however, the higher 

 group seems to have been directly derived from the earliest and 

 most generalized members of the preceding group, not from the 

 specialized members that flourished at the time of its dominance. 

 If, therefore, the Vertebrates originated from Arthropods, their 

 direct ancestors must have been early generahzed forms which 

 there is little hope of discovering among fossils. 



Although so little is known of their organisation, it seems 

 proliable that the Ostracoderms are lower in rank than the true 

 fishes, and most nearly related, among surviving animals, to the 

 Marsipobranchs. Dr. Gaskell has added to this probability by his 

 researches on the Ammocoete. His comparison of tlie structure 

 of the dermal head-shield in the Upper Silurian Aachenasjns with 

 that of the more deeply seated plate of muco-cartilage in the 

 Ammocoete, is particularly striking and interesting. 



Most of the Ostracoderms have a remarkable superficial resem- 

 blance to the contemporaneous Arthropods of the Eurypterid 

 group, being adapted for a similar mode of life on the sea-bottom. 

 A few, however, are laterally compressed and as gracefully fusi- 

 form as swiftly-swimming fishes (e. g., Birl-enia) ; and that these 

 had a wide geographical distribution in Upper Silurian times is 

 shown by the recent discovery of a fragment (named Ctenopleuron 

 nerepisense by G. E. Matthew) in New Brunswick. 



The supposed discoveries in Ostracoderms of appendages com- 

 parable with those of Arthropods, are due entirely to faulty 

 observation or misinterpretation. There is nothing more than a 

 normal branchial chamber on each side of the cranial region in 

 genera such as Cqyhalasins, Pteraspis, Cyathaspis, and Tremataspis, 

 where the skeleton can be well observed. The so-called paii-ed 

 appendages ascribed to the trunk of Cephalaspis by Prof. W. 

 Patten, are merely the scales which project along its sharp 

 angulation on each side. 



