GENERA PRODUCTUS AND STROPHALOSIA. 
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I have moreover ascertained that the American forms of P. costatus 
have often a well defined area on the ventral valve, and a narrow area 
on the dorsal valve, extending nearly the entire length of the hinge-line ; 
and in some specimens, this is as distinct as the area of Strophodonta or 
of Chonetes. There is of course a foramen, which is open in the examples 
observed; though in Mr. Davidson’s figure of P. semireticulatus, cited 
above, the foramen is closed by a deltidium. The specimens of ventral 
valves of Productus, which I have been able to examine, have no true 
teeth or extensions from the margins of the foramen; but sometimes a 
callosity extends obliquely backward and unites with the interior of the 
shell, being in fact as much a representation of dental lamellae as we 
have in some species of Strophodonta.* The accompanying figure 8 illus¬ 
trates the hinge-area of P. costatus; and figure 7 is of the interior of a 
dorsal valve, showing the muscular and vascular impressions. 
7 - 8 
Fig. 7. Productus costatus. Fig. 8. Productus costatus. 
Interior of the dorsal valve to stow the reniform Dorsal view of a specimen showing 
vascular impressions, cardinal process and the the hinge-area, 
oblique callosity C. 
Thus we see that 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 characte¬ 
ristic of Strophalosia. 
In glancing at the species of Productidse illustrated on Plates 28, 24, 
* See Strophodonta demissa, S. reversa, S. nacrea and others. 
