LAMBLLIBKANCHIATA. 493 



Ambonychia afflnis.] 



more prominent, giving it a more erect appearance than any of the other species 

 referred to the genus. Nor can I find that A. orbicularis ever has concentric undu- 

 ations. 



Formation and locality.— Middle Galena, Weisbach's dam, near Spring Valley, Minnesota; also in 

 Carroll county, Illinois. 



Mus. Beg. No. 8343. 



Ambonychia amygdalina Hall. 



PLATE XXXV. FIGS. 8 and 0. 



Ambonychia amygdalina Hall, 1847. Pal. N. Y., vol. i, p. 165. 



None of the specimens seen by me preserve the surface characters well enough 

 to prove that this species was provided with radiating lines. Obscure traces of such 

 striae are to be made out on one of the casts of the interior, but the evidence is not 

 sufficient for me to assert that they are what they seem. Still, it is highly probable 

 that radiating striae will be found on perfect specimens, in which case the species 

 would stand very near A. bellistriata, diftering from it, so far as we can now see, 

 chiefly in its greater size, less incurved beaks, flatter anterior side and less angular 

 postero-cardinal margin. 



The anterior lobe is longer, more sunken and less sharply defined in this species 

 than in the others here referred to Ambonychia. 



Formation and locality. — Middle Galena of Goodhue county, Minnesota. The New Yorlc type of 

 the species is credited to the Trenton limestone at Adams, Jefferson county. Billings also catalogues 

 the species from the same horizon in Canada. 



Genus CLIONYCHIA, Ulrich. 



Ambonychia (part.) Hall, 1847. Pal. N. Y., vol. i, p. 163. 



Clionychia, Ulkich, 1892. American Geologist, vol. x, p. 97; Clionyehia, Millee, 1892. First Ap- 

 pendix, N. A. Geol. and Pal., p. 699. 



Shells equivalve, moderately convex, subalate posteriorly; beaks terminal, com- 

 paratively small, not very prominent and but little incurved. Cardinal line straight, 

 rather long, forming an angle of less than 90° with the anterior side. Surface marked 

 concentrically only. No byssal opening, the margins closing tightly all around. 

 Muscular impressions situated in the postero-cardinal third, large, bilobed, the 

 lower lob'e much larger than the upper. Pallial line simple, extending from the 

 posterior adductor to the rostral cavity. Hinge plate of moderate strength, without 

 cardinal or lateral 'teeth, excavated longitudinally for a linear ligament. Upper 

 part of anterior edge thickened, producing a more or less well-marked impression 

 in this part of casts of the interior. Anterior pedal muscle attached a short distance 

 behind the beaks. 



Type: Ambonychia lamellosa Hall. 



