1863.] DAVIDSON NOVA-SCOTIAN BKACHTOPODA. 165 



to distinguish them ; and that, in addition to these, a species of 

 Bellerophon and a Trilobite of the genus PJiillipsia have been found 

 by Messrs. Meek and Hayden in the Permian rocks of New 

 Mexico, &c. 



Although palaeontological research is not in so advanced a state in 

 America as in Europe, and though the species of that continent will 

 require to be very carefully studied and compared with those of 

 Europe before being definitely recognized, still it is probable that the 

 relations between the Carboniferous and Permian deposits of the 

 States of Kansas and New Mexico are very close, and that there ex- 

 ists no marked line of break between them. A single investigator 

 might be mistaken ; but it is hardly likely that Messrs. Meek and 

 Hayden, Swallow and Hawn, and Shumard and Newbury, who have 

 examined for themselves, should all be in error ! 



The species recorded by these American geologists and palaeontolo- 

 gists as being common to the Carboniferous and Permian periods are 

 the following; but I append their list with the greatest possible 

 reserve on my part, and solely on their authority, as it is certain 

 that several of their identifications will require revision prior to 

 being definitely admitted : — 



1. Productus semireticulatus, Martin. 9. Orthisina Missouriana, Swallow. 



2. Rogersii, Norwood cf Pratten. 10. Rlrynchonella Osagensis, Swallow. 



3. sequicostatus, Shumard. 11. Terebratula (?) subtilita, Hall, 



4. Spirifera camerata, Morton. 12. Myalina recta, Shumard. 



5. plano-convexa, Shumard. 13. subquadrata, Shumard. 



6. pectinifera, Sow. 14. Kansasensis, Shumard. 



7. Chonetes Flemingii, Norwood §* Tr. 15. Allorisma Minnahaha, Swallow. 



8. Orthisina urnbraculum, Buch. 16. Naticopsis Pricei, Shumard. 



Dr. Shumard mentions also that Productus CaThounianus, Swallow, 

 has been found in both formations. 



However imperfect may be the lists I have given of the recurrent 

 species, it cannot be denied that, both by their genera and species, the 

 life -groups of the Carboniferous and Permian periods are closely re- 

 lated, and that although the Permian epoch possesses a large propor- 

 tion of species peculiar to itself, yet the general aspect of its fauna 

 greatly resembles that of the Carboniferous period. 



The immense material now in hand is piled up and carefully 

 stored, but we have not yet had time to sort our treasures. The time 

 and research hitherto required in the preparation and illustration of 

 the Monograph entrusted to my care by the Palaeontographical 

 Society, and the necessity of keeping my collections for that purpose 

 geologically and geographically arranged, have retarded the special 

 investigation of the class I am desirous of attempting in order to re- 

 spond to the request made me some time ago by Mr. Darwin, namely, 

 " that I should endeavour to exemplify and work out in some detail, 

 and with some single group, the theory of descent with modification, 

 so as to test the value of his theory." It appears to me, however, 

 preferable, and more advantageous to the point in view, that I should 

 continue and complete my Monograph prior to rearranging my whole 

 collection in a purely zoological order, and irrespective of all geolo- 

 gical divisions. By this last operation, all the forms that most re- 



