208 Mr. J. W. Kirkby on the Recurrency 



more in harmony with sound classification to employ a term 

 indicative of less magnitude than " system " in speaking of the 

 Permian strata. This some geologists already do, and, among 

 others, Sir Roderick Murchison, who, in his later contributions 

 to science, refers to the Permian strata as a group rather than a 

 system*. But I would further observe that, in adopting a divi- 

 sional term less likely to lead to wrong impressions in respect 

 to the importance of the Permian group of rocks, it does not 

 follow that the group should lose its distinctive appellation by 

 being considered but a formation or subformation of the Car- 

 boniferous system. I do not advocate this in the least, being 

 satisfied that it would tend to confuse rather than improve 

 geological classification ; for, notwithstanding its close palseonto- 

 logical relations with the last-named system, it would be useless 

 denying that its fossils are sufficiently peculiar to warrant the 

 employment of the special designation of Permian. 



In a paper on Carboniferous recurrent species lately published 

 in the ' Edinburgh New Philosophical Journal ' (vol. xiv. pp. 37- 

 45, & vol. xv. pp. 251-253), Professor William King objects to 

 some of the identifications of the Brachiopods of the preceding 

 list. His objections are chiefly confined to the identification of 

 Spirifera Urii with Sp. Clannyana, King, though other species 

 are made the subject of remarks. Considering the position that 

 Prof. King holds as an authority on species, it will perhaps be 

 well to notice the grounds of his objections, though I shall do 

 so somewhat briefly. 



Spirifera Urii and Sp. Clannyana. — In respect to these shells, 

 Prof. King maintains that they are distinct species, and that 

 neither the one nor the other was common to Carboniferous and 

 Permian faunas. In support of these opinions, he says that 

 " Urii differs from Clannyana in being a wider shell ; it has an 

 umbone more incurved ; the area of the small valve not so deep ; 

 the small valve flatter, and more excavated, as it were, towards 

 the posterio-lateral angles; the spines decidedly less numerous, 

 and the median sulcus more pronounced in both valves." With- 

 out disputing the existence of these peculiarities in some exam- 

 ples of Sp. Urii, I must still remark that in others, which I have 

 examined myself, there are no such differences to be observed. 

 In these specimens there exist an agreement of general form, 

 relative length and width, convexity and sulcation of valves, 

 size and shape of area, incurvation of umbone, and spinosity of 

 surface, with well-preserved individuals of Sp. Clannyana, that 

 could scarcely have occurred had they been distinct species. In 

 fact, there is no character, that I can see, on which to separate 



* Siluria, 3rd ed., and " On the Inapplicability of the Term Dyas," &c. 

 Philosophical Magazine, S. 4. vol. xxiii. p. 65. 



