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. obtusiplicata, Hall, of the Niagara 

 group, and it may prove that R. Indianensis, R. negleda, 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 



*Thi8 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. 



