BRracuiopopA. 825 
Rhynchotrema, Hall. 1860. 
(Plate 42, figs. 12-16.) 
Shells large, thick, often gibbous. In mature conditions the 
deltidial plates are of great size, thickened and coalesced with 
the bottom of the valve, their outer surface being concave. The 
pedicle-passage encroaches upon the substance of the valve, the 
foramen lying behind the apex and the passage itself inclosed by 
the thickened deltarium. 
‘The teeth rest upon the thickened lateral walls of the valve, 
and there appears to have been no development of dental lam- 
elle unless it was at a very early period in the life of the 
individual, 
In the brachial valve there is a thickened median septum which 
may extend for more than one-half the length of the shell; and 
it is upon the posterior extremity of this that the slender median 
cardinal process rests. This delicate apophysis is frequently dis- 
torted to one side or the other. The bases supporting the crura 
are divided by a very narrow median cleft, and are remarkably 
broad and stout, abruptly deflected to the deep dental sockets. 
The crura take their origin from the central portion of this com- 
paratively broad hinge-plate, instead of from the margins of the 
dental sockets, as is usually the case in the paleozoic rhynchonel- 
loids. The structure of the hinge apophyses in both valves is a 
persistent character, while the peculiarities of the deltarium are 
variable with age and external conditions. The muscular impres- 
sions are usually strongly developed, there being beneath the del- 
tidial plates a deep scar of the pedicle muscle, while the adductor 
impression on the pedicle-valve is often very marked. The 
adductors of the brachial valve and the diductors of the pedicle- 
valve are more or less distinctly defined. 
Type, Rhynchotrema capax, Conrad (sp.). 
Distribution. Lower Silurian. 
Rhynchotreta, Hall. 1879. 
(Plate 42, figs. 17-21.) 
Trihedral, strongly plicated shells with fold and sinus normal, 
in adolescent and mature stages; long and broad cardinal slopes; 
beak erect, acuminate and produced on the pedicle-valve. Fora- 
men at maturity apical, its upper margin encroaching on the 
104 17 
