GENERA PRODUCTUS AND STROPHALOSIA. 147 



while the disposition of the reniform vascular impressions of the dorsal 

 valve is a distinctive feature. 



In reference to the Genus Pkoductus, Mr. Davidson remarks as fol- 

 lows : 



" All well authenticated species of Productus, hitherto examined, have shown 

 " themselves to be edentulous ; but whether this character was general and 

 " without exception, may remain a question for further consideration : anyhow, 

 " the dorsal valve must have turned on its hinge-line with as much precision as 

 " in Chonetes, which possessed regularly articulating teeth. It has been often 

 " asserted and believed that Productus might be distinguished from its subgenera 

 " by the total absence of an area ; and although this is the prevalent character 

 " of the genus, still in certain species, such as P. sinuatus, a perfectly developed 

 " area is generally present in the ventral valve. There exists also an occasional 

 " tendency to the formation of hinge-area in several species ; as may be seen, for 

 " example, in the remarkable example of P. semireliculatus, of which a repre- 

 " sentation is given in Plate xliii, fig, 5."* 



These observations of Mr. Davidson are of later date than his " Intro- 

 duction to the Natural History of the Brachiopoda." 



One of the important features of distinction between Pkoductus and 

 Stbophalosia has been stated to be the different disposition of the reni- 

 form vascular impressions in the dorsal valve. The accompanying figures 

 represent this character in two species of the latter genus, as given by 

 Mr. Davidson in his Introduction. 



FlO. 8. SlEOPHAlOgIA OOLDfTMIl. FlO. 4. StEOPHALOSIA MOBEIglASA. 



In examining the species occurring in the higher formations of the New 

 York rocks, which I had supposed might be referred to the Genus Stbopha- 



• Monograph of British Carboniferous Brachiopoda, p. 136. 1861. 



