330 PALAEONTOLOGY OF NEW YORK. 



vonian period the incipient manifestation of the productidian type, which be- 

 came 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 character of its prototype in the 

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

 ized 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." 



From the Carboniferous Produdi Dr. Waagen proposed in 1884 to separate a 

 group characterized by a prominent internal ridge situated just within the 

 margins of the valves. To this group he gave the designation Marginifera, 

 and described it in the following terms : 



" The shells which I consider as belonging to the present genus are always 

 rather small, and never attain any considerable dimensions. In their external 

 appearance they are absolutely like Produdus, so long as the shell is not bro- 

 ken, but as soon as the shell-margin is removed, which very easily happens, 

 the difference comes to light. The cause why the shell-margin so very easily 

 breaks off is a thick, prominent shelly ridge, placed vertically on the internal 

 surface of the dorsal valve, and by which the visceral part of that valve is girt. 

 In the ventral valve a similar ridge is developed within the wings only. In 

 this way the visceral part of the shell is perfectly chambered off from the 

 remainder of the shell. These pi'ominent concentric ridges are sometimes 

 finely striated and crenulated ; sometimes smooth. The other internal charac- 

 ters are in all the specimens at my disposal (except in Marg. typica, W., where 

 the description will be found) very indistinct, but on the whole they seem to 

 be similar to those of Produdus. 



" This strange chambering off of the visceral part by means of proper pro- 

 jecting ridges seems to me perfectly sufficient for the generic distinction of 

 these forms. Certainly it is as well worthy of notice as the existence of an 

 area in Aulosteges or the like." (Salt-Range Fossils, p. 713.) 



