2 proceedi]st;s or the national museum vol. to 



line is long and gently arcuate, being about two-thirds the length 

 of the shell. The anterior margin is short, being about one-half the 

 length of the hinge. The margin is nearly straight for about one- 

 half its length and then curves backward to meet the ventral margin. 



Tavaodon is referable to the family Megalodontidae. It seems 

 most closely related to a new genus from the upper Silurian of 

 Alaska, the two standing apart from either Megalomus or Megalodon. 

 From the Alaskan genus Tmmodon difl'ers in the larger anterior 

 muscle, the smaller opisthodetic ligament, the great development of 

 lateral teeth, and in its general form and contours. In its general 

 form Taiiaodon bears a close resemblance to the Cyrtodontidae. 



The greatest dimension of the shell is in the diagonal line con- 

 necting the umbones with the posterior margin. This line is marked 

 on the exterior by a pronounced angular shoulder. The hinge line 

 is long and the anterior margin short, giving the shell a long narrow 

 obliquely inclined outline, viewed from the side. The umbonal 

 area is highly arched and the beaks prominent. They are incurved 

 toward the anterior end of the shell. The umbonal ridge noted above 

 is almost carinate in the anterior portion and persists as a strongly 

 marked angular shoulder to the posterior margin. From this ridge 

 the shell drops off abruptly to the dorsal margin and more gradually 

 in the other directions. 



The hinge plate is wide and massive. In the prepared specimen of 

 the type species there are eight well-defined cardinal teeth. These 

 are arranged fanwise, the tooth lying nearest the anterior margin 

 being subparallel to it and the one farthest removed being subparailel 

 to the dorsal margin. This group of cardinal teeth is followed by 

 a series of posterior lateral teeth. In the type species the latter are 

 three in number and are unusually long and massive. The passage 

 from cardinal to lateral teeth is so gradual that a division into two 

 grouj)s must be an arbitrary one. The adjacent teeth of the two 

 groups overlap and lie parallel, the tooth chosen as the first lateral 

 having its inception far forward near the anterior margin. 



There is a large, well-defined parivincular opisthodetic ligament 

 which extends from the beak backward for about one-half the length 

 of the hinge plate. The hinge plate is grooved for the reception of 

 the ligament, giving the latter a lenticular cross-section. The poste- 

 rior muscle scar has not been seen. The anterior muscle is large. 

 The scar is situated directly beneath the group of cardinal teeth and 

 lies on a built-up platform. The pallial line is simple so far as seen. 

 No pedal scar seems to be present. 



TANAODON LOUDERBACKI, new species 



This, the type and oidy known species of the new genus Tanaodon^ 

 was collected by Prof. George D. Louderback in the Middle Devonian 



