220 Annals of the Carnegie Museum. 



In that paper no real generic characters were brought out, the name 

 being proposed because of the unusual appearance of certain specimens 

 from Iron Ridge, Wisconsin, in which the deltidial plates were pre- 

 served. In part 2 of Volume VIII of the "New York State Paleon- 

 tology," 1895, Hall and Clarke further elucidate the characters of the 

 genus. They mention the absence of dental lamellae, the presence of 

 a thin linear cardinal process, and state that the crura arise from the 

 cardinal portion of a comparatively broad hinge-plate, instead of from 

 the margins of the dental sockets. The type is Rhynchotrema capax 

 (Conrad). 



The distinctive characters of CamarotcecJiia are stated by the authors 

 of the genus in the following words: 



"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. There is no cardinal process. In the pedicle 

 valve slender vertical lamellae support the rather small teeth and ex- 

 tend well into the cavity of the valve, enclosing a deep and narrow 

 pedicle scar." (Paleontology New York, Vol. VIII, pt. 2, p. 190.) 

 The type of the genus is Rhynchonella congregata Conrad, which usually 

 occurs as internal casts in sandy shale. 



Winchell and Schuchert, who wrote at about the same time as 

 Hall and Clarke, employ the name Rhynchotrema for all the "early 

 rhynchonelloid species having a prominent cardinal process." (Pale- 

 ontology of Minnesota, Vol. Ill, pt. i, p. 458.) 



The writer has before him some very good internal casts of Rhyn- 

 chotrema capax, and some silicified shells of R. incBquivalve. In all 

 of them the teeth of the pedicle valve are suported by thin lamellae. 

 In all the brachial valves there is a low median septum which divides 

 under the beak, producing a small spondylium as in Camarotcechia. 

 One of the silicified specimens shows a very thin cardinal process, 

 and the casts of R. capax show the former presence of this process 

 by a distinct groove. The presence or absence of the cardinal process 

 seems, then, to be the only way of separating the two genera. 



In the sandstone at Aylmer, Quebec, internal casts of Rhynchonella 

 orientalis Billings are very abundant and often well preserved. These 

 casts show distinctly the dental lamellae of the pedicle valve. In 

 the groove which is the impression left by the median septum on the 



