4 BRITISH OOLITIC AND LIASIC BRACHIOPODA. 



lence of characters by which we isolate Families or Groups from the remainder of their 

 class. 



However much we may feel inclined to dispute the existence in Nature of genera and 

 species, as an abstract proposition in the Philosophy of Zoology, the admission of these 

 terms, and of the ideas we are accustomed to associate with them, are essential to the pro- 

 gress of science. This being the case, we ought at least to found genera in a uniform 

 manner, and to equalize as much as possible the value of the characters upon which they 

 are based. If, among the conchiferous molluscs, some trivial modifications in the position 

 of the processes which protect their valves from dislocation be admitted, as of generic 

 importance, we surely ought not in the Brachiopoda to place together shells of such 

 different structure, as we remark in T. Carnea and T. Concentrica. M. Deshayes formerly 

 admitted very few genera in the Brachiopoda ; but from communications made by him to 

 me, I believe he now considers that a larger number are required from our present more 

 extended knowledge of the differences presented by the internal characters of the shell. 

 The value of these aids in the discrimination of genera is strongly insisted upon by 

 M. Deshayes in his great work upon the Paris Basin, where he states M. Blainville was 

 one of the first to remark, that in the recent Terebratulse the Apophysary lamella which 

 support the soft parts, present peculiar forms in each species, and adds, "Ce serait done par 

 ce moyen que Ton pourrait determiner rigoureusement les nomb reuses especes du genre." 



For many years our researches have been bent upon the discovery of those internal 

 characters; as we feel convinced that by them alone a rational and permanent classification 

 will in time be arrived at. Through our exertions, and the help of many kind and zealous 

 friends, we have been able to examine the internal structure of most of our British species 

 which disposes us to admit the following genera among them : — 



l. 



Lingula. 



5. Leptsena. 



2. 



Orbicula. 



6. Spirifer. 



3. 



Crania. 



7. Terebratula. 



4. 



Thecidea. 



8. Terebratella, 





9. 



Rhynchonella. 



Many more have of late been proposed among our Oolitic species, but which we hope 

 to discuss in the general introduction, and shall merely here observe, that the greater or less 

 length of a simply attached loop in Terebratula cannot be made use of as a generic 

 character, especially when there exists no other distinctive points. The length of the loop 

 maybe used as sectional, round which we can group certain species; but who, with any 

 degree of confidence, would place in distinct genera such shells as Ter. Cornuta, quadrqfida, 

 numismalis, ohovata, digona, ornithocephala, &c, which have a simply-attached loop extending 

 to near the frontal margin of the valve, and those such as Ter. punctata, perovalis, Max- 

 illata, intermedia spharoidalis, Coarctata, &c, the loop of which, simply attached to the 

 crura, only extends to less than half the length of the valve, while in others, such as 

 Ter. Carnea, Dyphia, &c. ; the same process attached to the crura does not extend to more 



