LEPIDOSIREN. 169 



ing a well marked line of demarcation between the coarse-tubed 

 and the fine-tubed dentine. The interspaces of the medullary canals 

 are occupied by a clear substance and by moss-like reticulations of 

 fine calcigerous tubes, which appear to be more sparing in number 

 than in the teeth of the sphyrsena or shark. The calcigerous tubes 

 of the external dentine run nearly parallel to each other, and vertically 

 to the external surface of the dental plate through about two thirds 

 (Jf the thickness of the external hard substance ; they then bend and 

 cross each other in a manner very similar to those of the external 

 layer of dentine in the teeth of the Lepidotus, Phyllodus, &c. 



In the process of attrition this external dense substance is worn 

 away from the upper surface of the dental processes in the lower jaw, 

 exposing the softer osseous, or medullary substance of the tooth ; in 

 this state the dental plate offers an analogy to the incisors of the 

 rodents, a posterior softer substance being sheathed by an anterior, 

 denser layer ; and an external sharp edge is similarly kept up by the 

 unequal wearing away of the two substances. The progressive waste 

 at the upper surface of the dental plate would appear to be met 

 by a corresponding addition of new material to its lower part. 



In the structure here presented to our observation we have a 

 condition of the dentine which has hitherto been met with only in the 

 class of fishes ; and the form, the extent, and the continuity 

 of the dental armature excepted, this part of the Lepidosiren 

 closely corresponds with that particular modification of the dental 

 structure which we have seen to be most eminently characteristic 

 of the class of fishes. The Sauroid character, for example, which 

 as in the Lepidosteus, pervades the air-breathing organs, and 

 which, as in the Polypterus, is traceable in the intestinal canal of 

 the Lepidosiren, is not at all manifested by the dental system : and 

 neither in the modified bone which forms the basis of the tooth, nor 

 in that coarser bone of which the jaw is composed, is there the 

 slightest trace of radiated cells or corpuscules ; the other parts of the 

 skeleton exhibit a similar ichthyic condition. 



The test of the affinities of the present paradoxical genus, afforded 

 by the microscopic examination of the teeth, gives additional con- 

 firmation to the views which I have already maintained from argu- 



