TEETH AND SPINES. 45 



teeth, and ready to replace them as soon as they 

 fall out. 



To pursue this subject further would be to 

 overstep the aim of this little work. Those who 

 may have found the facts herein set down inter- 

 esting will find that interest increased twofold 

 by an examination of the actual specimens such 

 as are displayed in the series of beautiful pre- 

 parations in the Natural History Museum of 

 London. To compare side by side the tooth 

 and the scale, and to trace the infinite grada- 

 tions leading from a tiny scale-like tooth to the 

 great triangular cutting tooth, as can be done 

 in many sharks, is a lesson in transforma- 

 tions that will not be soon forgotten. So great 

 is the difference between the two extremes that, 

 if they were examined apart from the inter- 

 mediate forms, they would be set down as teeth 

 of totally distinct species. This is a mistake 

 which has actually occurred in the identification 

 of the teeth of fossil fishes and we can well 

 understand it ! 



Spines, like teeth, are for the most part 

 modified scales. It is believed that the spines 

 which occur immediately in front of the dorsal 

 fins, as in the dog-fish for instance, owe their 

 existence to the fusion of the shagreen, or scales, 

 covering the front of these balancing organs. 

 They begin by forming a dense " cut-water" for 

 the support of the fins, and ultimately acquired 

 an independent position in the body. Fin-spines 

 play an important part in the study of fossil 

 fishes. Indeed, the spines are often the only 

 record we have left. Some of these fossil fin- 



