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 lirmly 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 



