i 



attenuate in casts, generally riming slightly forward, projecting l.ut little above 

 the hinge and incurved, Iteueath them the anterior side of caste present* 



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

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

 Surface, specially near the free margin-, marked with numerous, unequally 

 tnlmted concentric lines of growth, having the appearance, even on the caste, of 

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

 the ligamontal area wide and faintly striated. Mu-cular scar bilobed, situated almost 

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

 in- 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 

 numeioi^ 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 inpo>sil.le 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 lamtllosa and A. <iit<mil<i. 

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



11. il 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. 

 Whitiield'.s figure of .1. >ittrmmta, (p. tit. 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. Imt 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 />/<// 

 Hall, hut care is required in separating it from the two species of Clionyckia next 

 described. 



Formation and locality. LOW.T Itlm- and Upper Ituff llniMtone*, BeJolt. Mineral Point and JaiMt- 

 \\ aril; Dixon, Illinois, and th.- upper part f Uu- Trenton limestone at Minneapolis and St. 



Paul, Minnesota. 



Hut. Reg. NOR. 6676, - 



NITHM, n. 



I l.ATE XXXV. riOS. U mad M. 



This form i> -.. much like the preceding c . lamdlosa) that it scarcely deserves 



specific recognition. Critically .iiinpare<l it i> found to tlilter in tint folio, 



