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 
STROPHALOSiiE, 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 work, from having themselves labored in a similar field, 
will appreciate the difiiculties to be encountered ; and they will only need to be assured of my earnest 
intention to dispose of the species 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 cause to modify in some instances the views formerly maintained, and I believe I have shown 
no leniency towards species proposed by myself on insutficient 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. De Koninck, iii his “ Recherches des Animaux fossiles,” has not recognized the Genus Stro- 
phalosia; and the Devonian forms similar to those of our rocks are by him placed under Productus. 
