4 PROCEEDINGS OF THE NATIONAL MUSEUM vol.71 



sions on the other valve. These are scattered more or less promiscu- 

 ously along the posterior portion of the hinge plate. 



Of great interest in the hinge structure of the genus and an out- 

 standing structure differentiating it from the genera with which 

 it might be confused is the presence of an unusually large opistho- 

 detic ligament. This extends from the beaks to the posterior extrem- 

 ity of the hinge line. The ligament lies in a deep trough which as 

 a rule is V-shaped in cross section. Specimens have been seen in 

 which the dorsal margins of the shell are somewhat incurved, giving 

 the ligamental fossa a suboval cross section. In moderately large 

 specimens the trough in which the ligament lies has a depth of 

 about 1 centimeter, while in a large fragmentary specimen it has a 

 depth of more than 2 centimeters. The walls of the fossa are 

 striated longitudinally. 



The posterior muscle scar is inconspicuous but appears to be 

 fairl}^ large and situated high up in the valve below the posterior 

 end of the hinge. The anterior muscle is relatively small and varies 

 in size in individuals of approximately the same size. The scar is 

 deeply impressed and lies at the lower anterior extremity of the 

 hinge plate, closely apposed to the group of anterior teeth. The 

 pallial line is simple. No pedal scar has been identified. 



Pycnodesina may well be referred to the Megalodontidae. In 

 this family there are but two genera with which Pycnodesvia need 

 be compared — Megalorrius and Megalodon. Megalodon is thor- 

 oughly known, but the structure of Megalomius has never adequately 

 been figured and described. The figure given by Hall ^ is inadequate 

 and somewhat misleading. Megalomtas and Megalodon are inti- 

 mately related and may only be separated on minor structural 

 features. 



Pycnodesma may readily be distinguished from Megalodon and 

 Mewalomiis^ figures of which may be seen on Plate 2, Figures 6 

 and 7. An outstanding character is the enormous development of 

 the ligament in Pycnodesma. The hinge plate in Megalodon and 

 Megalomus is a massive anteriorly situated platform sharply cut 

 off from other shell structures and extending backward as an atten- 

 uated strip. In Pycnodesma the hinge plate is a broad, well-defined 

 structure extending to the posterior end of the hinge. The cardinal 

 teeth in Pycnodesma are more numerous and less massive than in 

 Megalodon. In Pycnodesma they are subequal in size, fairly 

 straight and parallel. In Megalodon and Megalomus the cardinal 

 teeth are of variable form and variously curved and oriented. In 

 Pycnodesma the teeth lie at approximately right angles to the hinge, 

 while in Megalodon they are subparallel to it. This feature as 



iHall, James, Taleontology of New York, vol. 5, pt. 1 (2), pi. 52. 



