BRACHIOPODA. 329 



Devonian form, and, as already observed, it is essentially equivalent to 

 DE Verneuil's division of the Producii caperati; certain small species which ex- 

 tend into the earlier faunas of the Carboniferous still retaining the cardinal 

 area and teeth. It is to be noticed that these features are in all cases obscure 

 and frequently difficult to discern, but even the large species occurring in the 

 late Devonian (Chemung) and early Carboniferous (Waverly), and which 

 resemble more in size and expression the normal species of the later faunas, do 

 retain them. There is no reliable evidence that Productus, as we have used 

 the term, occurs in Devonian faunas, and there is little reason to doubt that, in 

 this country, it does appear as early as the Waverly group (P. Newberryi and 

 P. semireticulatus). 



There are some peculiarities in Productella which may prove of value 

 in classification. The cardinal process rarely shows a trilobation when viewed 

 from the posterior face, the bilobate character being about equally developed 

 on both sides ; the delthyrium is apparently covered on both valves ; the 

 muscular impressions of the brachial valve are very small, and their surface is 

 not dendritic ; the brachial ridges or reniform impressions are rarely retained, 

 if ever present. The existence of teeth in the pedicle-valve implies the pres- 

 ence of sockets and crural plates in the brachial valve. The latter are 

 divergent ridges nearly parallel to the hinge-line, and corresponding to the 

 thickened ridges lying just within the cardinal margin in most species of Pro- 

 ductus. The combination of all these features, though they may not be suffi- 

 cient to give to Productella a thoroughly valid biological basis as a strongly 

 marked and distinctly limited generic form, may nevertheless serve to con- 

 tinue the usefulness of the designation in distinguishing certain forms among 

 the barren mass of Productoid material, where individuality is feebly repre- 

 sented, or entirely lost in the multitude of forms. With our present knowl- 

 edge and views of classification it must be regarded that Productella and 

 Productus are members of a descending series and represent different stages in 

 the process of degeneration. In the discussion of the characters of Pro- 

 ductella, given in Volume IV of the Palaeontology of New York (p. 151), the 

 following observation was made : " It appears to me that we have in the De- 



