184 
PALEONTOLOGY OF NEW YORK. 
which passes out at the apex or above the apex of the valve, a groove on the 
lower side always extending thence to the apex. This area [deltidium] some¬ 
times shows a longitudinal suture line, but this feature is not always visible.” 
The original specimens from which this account was derived are now before 
us, and enlarged drawings have been made from them which will show more 
clearly than did the original wood cuts, the accuracy of the description. The 
solid process in the umbonal cavity of the pedicle-valve is the deltidial plates, 
which are of great size, and are cemented firmly to the bottom of the valve. 
The concavity of their surface must be due, in a large degree, to the obese 
growth of the valves which forced the apex of the brachial valve against the 
deltidial wall. In younger shells, therefore, we should expect to find this 
cavity less strongly developed. Frequently these deltidial plates are wholly 
detached, and where retained, as in specimens from Richmond, Indiana, and 
elsewhere, they are narrower, not meeting and enclosing the foramen beneath, as 
in the shells described above. The encroachment of the pedicle-passage upon 
the substance of the valve, is thus due somewhat to the individual conditions 
of the shell, and is analogous to the complete enclosure of this channel in old 
examples of Leptana rhomboidalis, Wilckens, to which reference has been pre¬ 
viously made. The teeth rest upon the thickened lateral walls of the valve, 
and there appears to have been no development of dental lamellae 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 distorted to one side or the other. The bases support¬ 
ing 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 comparatively broad hinge-plate, 
instead of from the margins of the dental sockets, as is usually the case in the 
palaeozoic rhynchonelloids. The structure of the hinge apophyses in both 
valves is a persistent character, while the peculiarities of the deltidium, as has 
been observed, are variable with age and external conditions. The muscular 
