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 prominent 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.) 
