GENERA PRODUCTUS AND STROPIIALOSIA. 249 



It would appear that the Genus Strophalosia has been founded upon 

 characters derived 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 Strophalo- 

 sicB, but rather among the Producti, to which they seem more nearly allied.* 



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 excessive 

 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 characterized 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 characters, 

 should be the foundation of generic description and determination ; and in 

 this view Productus possesses fewer characters, and, in the absence of 

 hinge-area, teeth and sockets, is less complete than the forms possessing 

 these appendages. Nevertheless the great development of forms and number 

 constituting the Genus Productus during the Carboniferous period, has 

 acquired for it the importance and distinction which it must retain ; and 

 any subdivisions proposed must have reference to the characters of this 

 genus as established. 



On a critical examination of the genera of the Family Productidae, we 

 shall find that Cho/tetes is distinguished by well-marked and important 

 characters : having an area on each valve and a row of spines upon the 

 margin of the area of the ventral valve, which is one of the distinguishing 

 external features, though it has been ascertained that spines sometimes 

 occur upon the body of the shell. 



The separation of the genera or subgenera Strophalosia and Aulosteges 

 from Productus is based upon less important differences. The former has an 

 area on each valve, a foramen in the ventral valve covered by a deltidium, 

 with teeth and sockets, while the reniform vascular imprints have a 

 different direction and termination. The latter has a wide area on the 

 ventral valve, a foramen closed by a pseudo-deltidium, without teeth or 

 teeth-sockets, and with the reniform vascular impressions extended far 



* Prof. DeKoninck, in his '^ Recherches sur les Animaux fossiles," has not recognized the 

 Genus Strophalosia; and the Devonian forms similar to those of our rocks are by him 

 placed under Productus. 



[Assem. No. 239.] 32 . 



