SCHUCHERTELLA 53 



siderable amount of irregularity; they are narrower and less deeply 

 impressed towards the beak, becoming regularly stronger distally; fine 

 radiating costs? cross the concentric corrugations and continue to the 

 shell margin, from three to five occupying the space of one millimeter, 

 and these costge are again crossed by still finer concentric markings. 



Remarks. This species differs from L. analoga in the absence of the 

 distinctly flattened posterior region which passes into the anterior and 

 antero-lateral border by an abrupt geniculate deflection of the valves ; 

 instead of this the valves are more regularly curved throughout, the sur- 

 face curving somewhat more rapidly towards the margin instead of 

 being abruptly deflected. Upon this species the fine radiating cost* are 

 also more strongly and more regularly developed, although no broader; 

 the concentric corrugations are less deeply impressed and a little less 

 regular in their development, and the fine concentric markings are more 

 conspicuous. In their occurrence the two species are not associated 

 together in the same fauna, and the present species is less commonly 

 seen than is L. analoga. 



Horizon. Kinderhook. 



Genus SCHUCHERTELLA Girty 



Description. Shell biconvex, hinge-line usually equaling or sometimes 

 a little shorter than the greatest width, the lateral and anterior margins 

 rounded. Pedicle valve with a moderately high cardinal area, with a 

 large delthyrium which is closed to the apex by the deltidium. The 

 cardinal area is differentiated into two regions, the outer or primary area 

 and the inner or secondary area slightly raised above the former, the 

 lines separating the tw r o regions pass from the apex of the beak to points 

 about midway between the delthyrium and the cardinal extremities upon 

 the hinge-line ; the entire area is marked by horizontal lines of growth in 

 addition to which the secondary area is marked by vertical strias which 

 become more noticeable with the wearing of the surface. Internally the 

 dental lamellae are reduced to mere thickenings of the inner surface of 

 the cardinal area at the margins of the delthyrium; a median septum is 

 absent. The brachial valve is a little less convex than the pedicle valve, 

 with a narrow cardinal area, and a short cardinal process. 



Remarks. The genus Schuchertella, as originally proposed by Girty 

 for those non-septate orthotetoid shells which Hall and Clarke referred 

 to Orthothetes, includes two distinct generic types, as has been pointed 

 out by Thomas. 1 The first of these, Schuchertella proper, as typified by 

 the genotype 8. lens, includes the lesser number of the Mississippian 

 species, the second and larger group of species being designated by the 



iMem. Geol. Surv. Gt. Brit., Paleontology, vol. 1, pt. 2, p. 92 (1910). 



