456 THE PALEONTOLOGY OF MINNESOTA. 
LPlatystrophia biforata. 
with minute granules radially and concentrically arranged, which when worn away 
presents a punctate exterior]. In the interior of the ventral valve a prominent 
hinge tooth exists on each side of the fissure [delthyrium] and is supported by 
strong dental plates, which enclose an elongated, oval, raised [in America, depressed, 
with an elevated outer margin] muscular cavity of moderate dimensions. In the 
interior of the dorsal valve no prominent cardinal process is observable, but two 
short brachial processes deviate from the extremity of the umbonal beak and on the 
outer side of these are situated the hinge sockets. The quadruple muscle forms four 
very distinct cavities, strongly margined and divided longitudinally and transversely 
by prominent cross-like ridges." These adductor scars are not nearly as well defined 
in American examples. 
The writers regret their inability to secure very young specimens of this species 
for the purpose of determining the ancestors or line of development. In several 
immature individuals it has been observed that in the early nealogic stage the beaks 
are strongly elevated, probably erect, and each has a very large open delthyrium, 
surface smooth at first, but gradually developing eight plications and a mesial sinus 
in each valve. The sinus in the dorsal valve is bounded by two elevations, which 
become plications, and between them is soon developed a single costa which imme- 
diately bifurcates. The four plications increase in strength and become strongly 
elevated as they proceed to the anterior margin, producing the conspicuous fold of 
this valve. 
This widely distributed and protean species has its beginning, in North America, 
in the Chazy group, and is found in all the geological horizons upwards and into the 
Niagara formation. The earliest individuals are small in size and have but few and 
simple costse. Such specimens are found in the Cincinnati group around Cincinnati, 
Ohio, and are probably to be regarded as the young of P. biforata, or of the various 
forms occurring there and designated by varietal names. In the Ohio valley it 
attains its maximum in number of individuals, variation and size. In succeeding 
horizons it becomes less numerous and assumes characters somewhat ancestral. 
Adult individuals, occurring at a given horizon and locality, will be found to be 
fairly constant. However, in younger or older rocks, variations are continually 
taking place, and if specimens are gathered promiscuously from various horizons 
at a locality in which the shells are common, it will seem as if there were no con- 
stancy whatever in the species. While in a restricted region there is considerable 
permanence in shape and number of costae, in the same geological formation other 
variable characters sufficiently fixed for specific use are wanting. The species is to 
be regarded as very persistent and capable of readily adapting itself to changes of 
environment. Among the Brachiopoda such forms appear to be long lived. Others 
