THE LOWER SILURIAN ROCKS. 653 



to the posterior extremity of the cardinal border. Umbonal ridge much 

 more distinct than on the exterior of the shell, being defined anteriorly 

 by a distinct sulcus. 



This species is widely removed from Modiolopsis modiolaris, Conrad, 

 with which collectors of Kentucky fossils have often confounded it. 

 Aside from the well developed cardinal teeth, the more oval shape and 

 rounded instead of sinuate basal margin, should under ordinary circum- 

 stances, be quite sufficient to separate them. The Modiolodon winchelli 

 (Cyrtodonta winchelli, Safford), of the Trenton of Tennessee, I considei 

 as closely related. The latter is distinguished, however, by its less regu- 

 larly ovate form, the shell being as a rule not so high and always more 

 produced in the post-basal region, and. less uniformly rounded at the 

 junction of the cardinal and posterior margins. The cardinal teeth also 

 are more oblique and situated almost entirely behind the beaks, while 

 one in each valve is stronger than the other. Occasionally, again, only 

 one tooth is developed in the left valve. 



Formation and locality: This shell is very abundant at numerous 

 localities in Boyle, Mercer, Anderson and Franklin counties in Kentucky. 

 Its vertical range is restricted and it is, therefore, a highly characteristic 

 fossil of the argillaceous limestone beds resting upon the massive crys- 

 talline limestone which has furnished so many remarkable forms of 

 cystidea and other echinodermata, and constitutes the base of the Tren- 

 ton group proper in Kentucky. I believe the species also occurs in a 

 similar position in Tennessee. 



Modiolodon oviformis, var. amplus n. var. 



Plate 53, Figs. 1 and 2. 



The above provisional designation is proposed for a form that is to 

 be found associated with M. oviformis at Frankfort, Kentucky. It dif- 

 fers, so far as the characters of the variety are shown in four specimens, 

 in the straighter basal margin, more distinct pallial line, and larger size. 

 The length of the four specimens varies between 60 mm. and 68 mm. 



Modiolodon subrectus, n. sp. 



Plate 53, Figs. 5 and 6. 



This species is known only from casts of the interior. Of six speci- 

 mens the largest is 47 mm. long and 27 mm. high, the smallest 29 mm. 

 long and 17 mm. high. Cardinal and basal margins nearly straight and 

 subparellal, diverging slightly posteriorly; posterior margin obliquely 

 subtruncate, obtusely angular above, most prominent and strongly rounded 

 in the lower half; anterior end short, small, the upper margin sunken 

 considerably beneath the dorsal outline. Beaks compressd, prominent 



