76 Royal Society. 



cartilage, that is to say, cartilage-cells in an ossified matrix. Even 

 the hard spines of the fins and of the skin of these animals are not 

 real hone, hut dentine, as was demonstrated long since by Agassiz 

 and Quekett. 



If now we sum up all that has been said, we arrive at the following 

 conclusions : — 



I. There exist three types of structure in the skeleton of fishes, viz. : 



1 . Type of the Selachii. 



The skeleton is formed of cartilage or ossified cartilage. 

 Selachii, Cyclostomi. 



2. Type of the Acanthopterygii. 



The skeleton is formed of a homogeneous or tubular osteoid 



substance, often of true dentine. 

 Teleostei (J. Milll.), with the exception of the greater 



part of the Physostomi (J. Milll.). 



3. Type of the Ganoidei. 



The skeleton is formed of real osseous tissue. 



Most of the Physostomi, the Ganoidei, and Sirenoidei. 



II. The exoskeleton follows in some respects the same laws as the 

 endoskeleton, and shows the following types : — 



1 . Exoskeleton formed of a homogeneous and fibrous osteoid 



substance. 

 Scales of the majority of the Teleostei. 



2. Exoskeleton formed of dentine. 



Spines of Selachii and scales of Plectognathi, and of Amphi- 

 sile, in part. 



3. Exoskeleton formed of real bone ; partly in association 



with homogeneous osteoid substance (ganoin) and 

 dentinal tubes. 

 Scales of Ganoidei, of Lepidosiren, some Siluroidei, of Mor- 

 myri, many Characini and Clupeini, also of Thynnus. 



In terminating this communication, I think it proper to mention 

 that the great liberality with which my friend Mr. Tomes of London, 

 and Professor "Williamson of Manchester, put their large collections of 

 microscopic preparations of teeth, bones, and scales at my disposal, 

 proved of great assistance in my investigations, and, accordingly, I 

 am only fulfilling an agreeable duty in now publicly expressing my 

 obligations to them. I am also greatly indebted to my friends Filippo 

 de Filippi of Turin and Henry Muller of "Wiirzburg, also to Dr. 

 Hyrtl of Vienna, and Dr. Peters of Berlin, who supplied me with 

 many of the rarer Mediterranean and foreign fishes. But, in order 

 that my observations may yield the results which may not unrea- 

 sonably be expected from them, I need more aid ; and as England 

 is the country in which not only the largest zoological collections of 

 fishes, but also the greatest number of microscopic preparations of 

 the hard tissues of recent and fossil animals, are to be found, I take 



