INTRODUCTORY. 



The present work is largely though not exclusively concerned with 

 the consideration of those distinctively carboniferous families of 

 ancient Selachians, the Cochliodontidae and the Psammodontidse. 

 Although in the majority of instances the classification now adopted 

 has necessitated the amending, amplifying, and curtailment of the attri- 

 butes ascribed to genera, it has not been deemed necessary to give a 

 detailed diagnosis of all the genera thus revised. In the latter in- 

 stances the descriptions of species will afford the necessary informa- 

 tion relating to the peculiarities of the genera to which they belong, 

 and a glance at the illustrations will often convey a vivid impression 

 of those distinctions. 



It has been impossible to avoid repetition of details in the notices 

 of the species, especially in the desire to render for each species a 

 comprehensive diagnosis by itself ; otherwise the descriptions would 

 mainly consist of comparisons of their distinctive features as con- 

 trasted with one another, and we have deemed it the safer plan to 

 err on the score of details rather than brevity, stating the facts 

 concisely as possible, but omitting none of evident consequence as 

 aids to a fair conprehension of their special characteristics and their 

 resemblances and distinctions compared with allied species. In no 

 similar investigation have so many and varied materials relating to 

 these interesting groups of Selachians been brought forward for 

 critical consideration at the same time. That the subject was some- 

 what involved may be readily understood by all students; but to 

 none so much as to the specialist can the actual state of things in 

 this relation be at all adequately appreciated. This has largely re- 

 sulted from incomplete data, and to some extent also to the lack of 

 familiarity with the association of forms in the same deposits. For 

 it must be confessed that in the very few exceptions these remains 



