GENERA PRODUCTUS AND STROPHALOSIA. 



247 



Neither of these species are known to have the apex of the ventral 

 valve truncated. In another species, with extremely gibbous and arcuate 

 ventral valve and deeply concave dorsal valve, the disposition of the reni- 

 form impressions is precisely like those illustrated ; and in none of the 

 species under investigation have we seen the least evidence of reniform 

 impressions like those of figures 3 and 4, which are illustrations of Permian 

 species of Strophalosia. 



It is clear, therefore, that this character could not be relied on for 

 distinguishing the genus, if we were to include in it our Devonian species; 

 for in the three species referred to, the imprint has the same direction and 

 termination as shown in Aulosteges, except perhaps that the muscular 

 scars may be a little nearer the hinge-line ; and in this respect it is not 

 very dissimilar to some species of Productus, as shown in P. costatus 

 (%ure 7).* 



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 StrophodontaA The accompanying figure 8 illustrates 

 the hinge-area of P. costatus; and figure 7 is of the interior of a dorsal 

 valve, showing the muscular and vascular impressions. 



Fig. 7. Fig. 8. 



Productus costatus. 



Interior of the dorsal valve, to show the 

 reniform vascular impressions, cardi- 

 nal process and the oblique callosity C. 



Productus costatus. 



Dorsal view of a specimen, showing the 

 hinge area. 



* Productus longispinus and P. pustulosus have a similar form of the reniform impres« 

 sions. See Davidson's Monograph of British Carboniferous Brachiopoda. 



^ f See Strophodonta demissa, S. reversa, S. nacrea and others. 



