GENERA PRODUCTUS AND STROPHALOSIA. 151 



that I approach the final disposition of the species of this character in 

 our formations.* 



It would appear that the Genus Strophalosia has been founded upon 

 characters derived entirely from Permian species, in which the interior 

 structure has been well preserved. Of the few European Devonian species 

 of this genus which are cited, I have seen no illustrations of the interior ; 

 and these have probably been referred to the genus from the generally 

 similar form, the presence of a narrow area, and minute teeth and sockets, 

 which are characters found in identical or similar forms in the American 

 strata. It is probable, therefore, that had the interior of these species 

 been observed, they would not have been placed among the Permian 

 SiROPHALOSiiE, but rather among the Producti, to which they seem more 

 nearly allied.f 



It appears to me that we have in the Devonian period the incipient 

 manifestation of the productidian type, which became modified in the 

 later Carboniferous period, where, with conditions favorable to its exces- 

 sive development, it has assumed extravagant forms and proportions, 

 but here and there indicating the characters of its prototype in the 

 presence of an area and foramen, among species of a genus which is cha- 

 racterized by the absence of these features. In the still later Permian 

 period, with changed conditions, we have an approximate return to the 

 earliest forms of the species, or to the prototype ; and in reality the 

 foundation of the genus lies in the Devonian forms which have been 

 referred to Strophalosia. 



It would appear that the presence, rather than the absence of charac- 

 ters, should be the foundation of generic description and determination ; 



• Those who have the best right to criticise the worli, from having themselves labored in a similar field, 

 will appreciate the difficulties to be encountered ; and they will only need to be assured of my earnest 

 intention to dispose of the specios without prejudice or partiality to previously expressed opinions. With 

 larger collections before me than I had when originally describing the species under the preceding genera, 

 I have seen canse to modify in gome instances the views formerly maintained, and I believe I have shown 

 no leniency towards species proposed by myself on insufficient materials ; but I am quite unable, at the pre- 

 sent time, to reduce the species of this type from these formations, to the limits indicated by European 

 authors who have examined American collections. 



t Prof. Dk Kohihck, in his " Recherches des Auimaux fossiles," has not recognized the Genus Steo- 

 rBAt,osiA ; and the Devonian forms similar to those of our rocks are by h'im placed under Pbodpctds. 



