LAMELLIBRANCHIATA. 521 
Modiolodon, 
Genus MODIOLODON, n. gen. 
Cyrtodonta (part.) SAFFORD, 1869, Geol. of Tenn.; Modiolopsis (part.), ULR1cH, 1890, Amer. Geol., 
vol, v. 
Ovate shells of the same general type as Modiolopsis and Modiomorpha, but hay- 
ing from one to three oblique cardinal teeth in each valve. 
Type: Modiolopsis oviformis Ulrich. 
The hinge in this genus is much like that of Ischyrodonta, Ulrich, and 1 might have 
placed the species under that genus were it not for the fact that their shells are of 
the same composition as those of Modiolopsis, while the shells of that genus are gen- 
erally heavier and of the more calcareous nature characterizing the Cyrtodontide. 
The development of distinct cardinal teeth is an important deviation from 
Modiolopsis, and 1 cannot see how any one could object to the generic separation 
of species possessing them. Surely, if Modiomorpha can stand, then Modiolodon must, 
for its claims for recognition are certainly better. This may be seen from the accom- 
panying sketches of the hinges of the three genera. 
Fig. 40. Hinges of Modiolopsis, Ifodiomorpha and Modiolodon. a, anterior half of hinge of a 
right valve of Modiolopsis versaillensis Miller, from the upper part of the Cincinnati group at Versailles, 
Indiana. 6, hinge of a right valve of Modiolopsis valida, a new species from the top of the Lower 
Silurian at Waynesville, Ohio. This species, though closely related to M. modiolaris, has a wider hinge 
plate than in any other species of the genus known. c¢, hinge of a left valve of Modiomorpha concentrica 
Conrad, sp., from the Hamilton of New York. d, anterior part of hinge of right valve of Modiolodon 
ganti (Cyrtodonta ganti Safford}, e and f, of right and left valves of Modiolodon winchelli (Cyrtodonta 
winchelli Safford), and g, of a left valve of Modiolodon oviformis Ulrich; all from the middle Trenton 
(“Orthis beds”) of Wilson county, Tennessee, and from specimens kindly given to the author by Prof, 
J. M. Safford. 
Modiolodon ganti and M. winchelli, two of the most typical species of this genus, 
were placed into the genus Cyrtodonta, Billings, by Prof. Safford. Aside from their 
different shell structure, they have no right in that genus, being without posterior 
lateral teeth, 
MopioLopON PATULUS, 7”. Sp. 
PLATE XXXVII, FIGS. 20—24. 
Shell of medium size, suberect, compressed convex, broad ovate, very inequi- 
lateral; anterior end very short, in the casts occupied almost entirely by the strongly 
elevated, lobe-like, anterior muscular scar. Hinge line short, the posterior part of 
