THE LOWER SILURIAN ROCKS. 671 



Genus ISCHYRODONTA Ulrich. 



Anodontopsis (part), Meek, 1871, Amer. Jour. Sci. and Arts, 3d ser., vol. ii, p. 299; 

 also 1873, Ohio Pal., vol. i, p. 141. 



Ischyrodonta, Ulrich," 1890, Amer. Geol., vol. vi, p. 173. 



Short or moderately elongate, thick bivalve shells, having small, an- 

 teriorly situated beaks, with the hinge line straight or arcuate and ex- 

 tended posteriorly. Hinge plate wide and strong, without posterior 

 lateral teeth, but with two strong cardinal teeth in the left valve, and 

 one large one, and occasionally a small, one on each side of it, in the 

 right valve.* Ligament internal, posterior to the beaks, linear, sup- 

 ported by from one to three subcardinal ribs. Anterior adductor im- 

 pression large, deep, subovate, sharply defined on the inner and upper 

 side by a ridge extending from the cardinal teeth to the base of the 

 scar. A small pedal muscle was attached to the under side of the hinge 

 plate immediately above the inner side of the anterior adductor scar. 

 Posterior muscular scar faintly defined, generally but little larger than 

 the anterior scar, situated a short distance beneath the posterior extrem- 

 ity of the hinge. Pallial line simple. Test thick, chiefly calcareous, 

 without the dark epidermis of the Modiolopsidm and Ambonychiidee. 



In casts of the interior the beaks are prominent and strongly com- 

 pressed, and a more or less well defined sulcus, corresponding to an in- 

 ternal thickening of the shell, extends from the umbones more than half 

 the distance to the center of the basal margin. 



Type : Ischyrodonta truncata Ulrich. 



The relations of this genus are clearly with Ortonella. The reader 

 will find them discussed in the remarks following the description of that 

 genus on the preceding page. Through Ortonella the genus is linked with 

 the Cyrtodontida, and in referring the species to this family I have been in- 

 fluenced in a great degree by the composition of their shells which 

 seems to be precisely as in Cyrtodonta and. related genera, and not as in 

 the true members of the Modiolopsidtz. Except for this fact it would be 

 a difficult matter to draw the line between Ischyrodonta and Modiolodon, 

 the general aspect of the shells and the dentition of their hinges being 

 practically the same in both. It may seem, therefore, that when casts 

 of the interior only are available it is not possible to discriminate suc- 

 cessfully between the two genera. This is, however, not so, since the 

 casts of every species of Ischyrodonta now known exhibit at least one 

 small feature that has not been seen in any species of Modiolodon nor in 

 any now referred to the Modiolopsid/z. Namely, the small pedal muscle 



* In the original description the arrangement of the cardinal teeth was in- 

 advertently reversed. 



