Xll PREFACE. 



the true nature and mode of development of a class of organs by 

 tracing the modifications of such through the entire range of the 

 series of animals to which it is peculiar, is, that, from the length 

 of time required to complete and arrange the extended series of 

 observations, partial glimpses and illustrations of the main con- 

 clusions sought to be estabhshed are published by authors who 

 are excited to pursue some limited branch of the subject, and 

 have leisure for following it out. The right of priority of original 

 observation thus affected, is, however, a matter only of personal 

 interest, and of small moment in comparison with the benefit which 

 science derives not only from the collateral and independent evidence 

 thus adduced, but also from the stimulus to further research emanat- 

 ing from the discussion of such right. Where the concurrent 

 investigations are liberally pursued in the spirit of truth, they ought 

 to produce no other feeling than that of friendly emulation. It is 

 with unalloyed pleasure that I have seen the investigations com- 

 menced in the first Part of the present Work, extended by the 

 beautiful illustrations of the microscopic structure of the Teeth of 

 Fishes in the later Numbers of the ' Poissons Fossiles,' of M. Agassi z, 

 in which most of my descriptions are verified,* and my indications 

 of the labyrinthic structure of the teeth of certain Fishes have 

 received direct illustration by the figures of the microscopic structure 

 of those of the Lepidosteus. In like manner the Memoirs of M.M. 

 Erdl, Bibra, and Duvernoy,! have confirmed and extended my 



* With regard to the Psammodus and allied extinct Fishes, in which the medullary canals 

 are affirmed by M. Agassiz to open directly upon the grinding surface of the tooth, it would 

 be as reasonable to suppose that the long vascular and sensitive pulp should be exposed upon 

 the working surface of the perpetually growing incisor of a Rodent. But apart from any 

 physiological objection to the opinion of the learned Ichthyologist of Neuchatel, I have made 

 new sections of the teeth of diflferent species of Psamraodonts, and have demonstrated their 

 perfect agreement with my descriptions, and with Plate 20, in the first Part of this Work, 

 to the satisfaction of Sir Philip Egerton, Mr. Stokes, Mr. Broderip, and other scientific 

 friends. All these specimens show that, as the grinding surface is worn down, the vascular 

 contents of the medullary canals have become calcified within a short distance from that 

 surface, and thus, in the existing Fish, were the cavities of the canals defended from the 

 effects of friction. 



f I regret that the pages in Part III, descriptive of the Teeth of the Insectivora were 

 printed oflF before I received the last memoir of M. Duvernoy containing his observations and 

 beautiful figures of the microscopic structure of the teeth of the Shrews. 



