248 TWENTIETH REPORT ON THE STATE CABINET. 



Thus we see tliat the species referred to Productus may have all the 

 characters of Strophalosia except the hinge- teeth and sockets, and the 

 disposition of the reniform impressions ; while the New- York species, 

 possessing teeth and sockets, have not the disposition of the reniform 

 vascular impressions which are given by European authors as characteris- 

 tic of Strophalosia. 



In glancing at the species of Productidae illustrated on Plates 23, 24, 

 25 and 26 of Vol. iv, Palaeontology of New- York, one cannot fail to be 

 impressed with the prevailing greater width on the hinge-line, and conse- 

 quent modification of form iQ the American Devonian species, as compared 

 with the Strophalosia of the Permian system in Europe. But while the 

 greater part, and perhaps all of these, have a narrow cardinal area and 

 small hinge-teeth and sockets, I conceive that their exterior character and 

 expression are much more like the true Productus than are the Permian 

 species of Strophalosia ; and one of the most marked features consists in 

 the conspicuous ears with rows of spines, as in Carboniferous Producti ; 

 while their anterior extension and highly arcuate forms assimilate them 

 much more with Productus than with Strophalosia, 



The Productidae of this character make their first appearance in the 

 American geological series in the Upper Helderberg group, and continue with 

 increasing numbers through the successive formations to the Carboniferous 

 limestone. There are a few individuals of two species in the Corniferous 

 limestone, and more in numbers of individuals and of other species in the 

 Hamilton group ; but it is difficult to find specimens in a coildition favora- 

 ble for satisfactory investigation. A few specimens have been obtained 

 entirely separated from the rock ; but nearly all are imbedded, with the 

 ventral valve adhering to the matrix so that the hinge and interior struc- 

 ture can rarely be seen ; while the dorsal valves almost uniformly adhere 

 by their exterior surface, leaving only their interior for examination. 



In the Chemung group, the specimens occur mostly as casts of the inte- 

 rior and impressions of the exterior, and these are frequently much 

 distorted. In some of the softer compact shales of this group, we find the 

 shell partially preserved, but still in such condition that the entire charac- 

 ters cannot be discovered ; and it is only in some fresh exposures of the 

 semicalcareous bands that we are able rarely to obtain specimens of more 

 natural appearance and better preserved surface. 



Under circumstances of so much difficulty, it is with much hesitation 

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

 formations.* 



* Those who have the best right to criticise the work, 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 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 insufficient materials; but I ani quite 

 unable, at the present time, to reduce the species of this type from these formations, to 

 the limits indicated by European authors who have examined American collections. 



