

667 



Perch are in fact the tubules of the osteoid substance ; 2, that 

 amongst the group of fishes in question, there are some which have 

 beautiful dentine in their skin-bones, e. g. Amphisile scutata and the 

 Ostracionts. 



To the foregoing remarks on the microscopic structure of the hard 

 tissues of fishes, I may add, that there also exists a third group of 

 fishes, in which the endoskeleton is composed only of common carti- 

 lage, or of cartilage with depositions of earthy salts, viz. the Cyclo- 

 stomi and Selachii. None of these fishes, not even the Plagiostomi 

 and Chimcera, possess real bone-cells in their hard parts ; for these 

 are formed only, as J. Miiller showed many years ago, by ossified 

 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 bone, but 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. Mull.), with the exception of the greater 



part of the Physostomi (J. Mull.). 



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. 



