190 
PALEONTOLOGY OF NEW YORK. 
they usually maintain a full trihedral contour with shallow pedicle-, and con¬ 
vex brachial valves, evincing little, if any evidence at maturity, of a reversal 
of the relative convexity of early growth, a feature so apparent in some of the 
other groups of the rhynchonelloids. Their distinctive characters, however, 
are internal; the median septum of the brachial valve is divided posteriorly in 
such a manner as to form an elongate cavity, which does not extend to the 
bottom of the valve. Each branch of the septum supports one of the lateral 
divisions of the hinge-plate, to which are attached the curved crural processes. 
In normal conditions of development the median interspace of the hinge-plate 
is not closed. The dental sockets, bordering the hinge-plate, are crenulated in 
the species which are assumed as representing the typical characters of the 
group. There is no cardinal process. 
In the pedicle-valve slender vertical lamellae support the rather small teeth 
and extend well into the cavity of the valve, enclosing a deep and narrow 
pedicle-scar. 
This is a group of shells highly developed in species, and eminently charac¬ 
teristic of the Devonian faunas, and hence the Rhynchonella congregata, Conrad, is 
designated as the type of the genus. This species is abundant in the condition 
of excellently preserved internal casts, in the sandy shales of the Hamilton group 
of central and eastern New York. The type of structure is, however, much older, 
probably as ancient as the early Trenton faunas of the Lower Silurian, where 
it seems to be represented by the species Rhynchonella altilis and R. plena, Hall, 
of the Chazy limestone.* In the succeeding faunas of the Silurian are 
R. fringilla and R. glacialis, Billings, from Division I, of the Anticosti series; 
R. aequiradiata. Hall, of the Clinton group; R. ohtusiplicata, Hall, of the Niagara 
group, and it may prove that R. Indianensis, R. neglecta, R. Whitii and R. acinus, 
Hall, from the same faunas of New York and Indiana, also belong here, though 
their external habit, i. e., small size, compressed or elongated valves, is not 
usual in this group. At the appearance of the Lower Helderberg faunas, with 
their multiplicity of rhynchonellids, this type of structure appears to have 
^t'This opinion is based upon serial transverse sections of the shells ; since no separated valves or satis¬ 
factory internal casts of these species have been obtainable. 
