ON THE TEETH OF PETROMYZON AND MYXINE. 635 



be more efficacious, and it would probably uot be difficult for 

 natural selection to replace the horuy tooth by its calcified 

 core. This would be the primitive dentine. 



We can imagine that at a later period the calcification 

 which first originated in the mesoderm of the pulp-cavity of 

 the wart afterwards extended into the Malpighian layer of 

 the epidermis, and this would constitute the primitive 

 enamel. 



In support of such a view it should be remembered that 

 among the dentines of the teeth of living and fossil forms we 

 can meet with every transition from the most irregular 

 calcified mass of vaso-deutine to the highly organised, fine- 

 tubed varieties. Also among enamels there can be found 

 every transition from the thinnest varnish-like layer of 

 apparently homogeneous calcareous matter, to thick layers 

 of enamel, consisting of striated prisms and interprismatic 

 substance. 



The teeth of the jaws in the primitive condition would 

 originate by a separate ingrowth (diagram 1) of epithelium 

 from the surface for every tooth. Such ingrowths are 

 frequently indicated externally by slight swellings, which 

 pei'haps represent the horny tooth, which according to the 

 hypothesis advanced phylogenetically preceded the calcified 

 tooth. An advance on this condition can be seen in the case 

 of the Pike, where frequently an enamel-germ buds out 

 from an older enamel-germ instead of from the general 

 epidermis. 



If the ingrowths of epidermis for the individual teeth 

 occurred close together or in contact along a single line 

 around the edge of the upper and lower jaws we should have 

 the beginning of the tooth-band (diagrams 2, 3, 4). 



If, now, all the new enamel-germs for the successional 

 teeth were regularly budded off from the preceding germs, 

 instead of only a few, as in the Pike, we should arrive at the 

 condition well seen in the lower jaw of embryo Scyllium 

 catulus (diagrams 5, 6). 



In an older embryo, and especially in the lower jaw, the 



