BRACH10P0DA. 
329 
Devonian form, and, as already observed, it is essentially equivalent to 
de Verneuil’s division of the Producti 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- 
