LAMBLLIBRANCHIATA. 495 



Olionyohia nitida.l 



attenuate in casts, generally curving slightly forward, projecting but little above 

 the hinge and scarcely incurved. Beneath them the anterior side of casts presents 

 a broad and often sharply-defined depression which, in extending downward, gradu- 

 ally dies out at or a little beneath a point midway between the base and the hinge. 

 Surface, especially near the free margins, marked with numerous, unequally dis- 

 tributed concentric lines of growth, having the appearance, even on the casts, of 

 being the edges of overlapping lamellae. Hinge plate rather strong, without teeth, 

 the ligamental area wide and faintly striated. Muscular scar bilobed, situated almost 

 entirely within the postero-cardinal third of the valve, Pallial line simple, extend- 

 ing up the anterior side apparently to the cavity of the beak. 



The form of this species seems to be quite variable, but after a careful study of 

 numerous specimens I have concluded that much of this supposed instability is due 

 to distortion through pressure. On the other hand, for the same reason, 1 found it 

 utterly inpossible to detect really normal specific differences between the specimens 

 which Hall in his original work and Whitfield in the later volume cited above have 

 separated as two species under the names Ambonychia lamellosa and A. attenuata. 

 According to my view the latter is founded upon specimens of the former whose 

 original form was changed by pressure acting so as to decrease the diagonal or the 

 vertical diameter of the valves, causing them to appear abnormally elongate. 

 Whitfield's figure of A. attenuata, (op. cit. plate V, fig. 6) represents, instead of the 

 left, most surely the right side of an, obviously distorted specimen. It is a little 

 surprising that a paleontologist of such wide experience as Prof. Whitfield should 

 have failed to observe the evidences of distortion, and more so still, that he should 

 mistake one valve for the other, especially of a specimen that preserves the posterior 

 adductor scars. These we know-are situated in the postero-cardinal third of the 

 valves, but his error leads him so far astray that he asserts without qualification 

 "they are situated near the anterior border of the valve." 



This species cannot be confounded with the associated Ambonychia planistriata 

 Hall, but care is required in separating it from the two species of Clionychia next 

 described. 



Formation and locality. — Lower Blue and Upper Buff limestones, Beloit, Mineral Point and Janes- 

 ■ville, Wisconsin; Dixon, Illinois, and the upper part of the Trenton limestone at Minneapolis and St. 

 Paul, Minnesota. 



Mus. Reg. Nos. 5676, 8314. 



Clionychia nitida, n. sp. 



PLATE XXXV, FIGS. 15 and 16. 



This form is so much like the preceding (C. lamellosa) that it scarcely deserves 

 specific recognition. Critically compared it is found to differ in the following 



