V ORIGIN OF THE BONY SKELETON 333 



the "membrane" bones of the roof of the mouth in Teleosts, and in the 

 tooth-bearing bones of Lung-fishes. Again there are present minute 

 enamel-tipped teeth scattered over the surface of the dermal bony 

 plates of Crossopterygians and various Siluroid Teleosts such as 

 Loricaria, Hypostoma, Callichtliys. 



Such facts as those just enumerated seem to justify the acceptance, 



as a working hypothesis, of the view that at least the more superficially 



-placed dermal bones of the Vertebrata have actually arisen in the 



course of evolution from the basal trabeculae or plates connected 



with placoid scales. 



Admitting this a further question presents itself. What was the 

 evolutionary origin of the more deeply situated masses of bony tissue, 

 for example those which replace cartilage ? Has the tissue which 

 gives rise to these gradually been infected with bone-forming activity 

 which has spread inwards from the skin ? Or has this bone-forming 

 power in the deeper tissues arisen independently ? It is in this 

 connexion extremely instructive to study the gradual spreading of 

 the irregular shreds of bony material from the tooth-base of a 

 Lepidosiren. They gradually spread onwards through the connective- 

 tissue matrix like crystals forming in a fluid, and there is no apparent 

 reason why such spreading should not continue for relatively great 

 distances, provided the necessary pathway of connective tissue is 

 present. It appears in fact thoroughly reasonable to regard the 

 deeper portions of the bony skeleton, like the more superficial, as 

 having arisen in evolution by the spreading inwards of bone-forming 

 activity from the skin. 



In considering this important morphological problem, the origin 

 of the bony skeleton, it must be borne in mind that the all-important 

 fact, which far outweighs all other evidence available, is that in the 

 Elasmobranchii, the group of gnathostomatous Vertebrates which is 

 admittedly the most archaic, the placoid scales are the only elements 

 of the osseous skeleton which! have as yet made their appearance. 

 There is no suggestion that the ancestors of existing Elasmobranchs 

 ever possessed a bony skeleton apart from the placoid scales. Con- 

 sequently in the Vertebrate groups which have been evolved sub- 

 sequently to the Elasmobranchs the bony tissue must either be a 

 further development of the bony basal plates of the placoid scales, 

 or else a new independent development. If the former view is shown 

 to have in its favour a reasonable degree of probability we are bound 

 to accept it as our working hypothesis until a better is suggested, for 

 it alone of the two views mentioned is really constructive, the 

 other offers no explanation but merely the negation of an explana- 

 tion. In the opinion of the present writer the reasonable degree 

 of probability has been amply demonstrated by the facts which 

 have been quoted. 



It is also necessary to avoid attaching too great importance 

 to the differences in detail which have arisen in the evolutionary 

 history of bony tissue under different circumstances. Such differ- 



