LAMELLIBRANCHIATA.. 491 



Arabonychia planlstriata,] 



be more nearly related to Ambonychia than either of the genera mentioned. Certain 

 it is that I find it more difl&cult to point out the distinguishing features than I did 

 in those cases. The general appearance of the shells of the two genera {Ambonychia 

 and Amphicoelia) is very similar, both in the matter of form and in their surface mark- 

 ings. The hinge also is very much the same in the two genera, the chief difference 

 being that the area is wider in Amphicoelia. The greatest difference, however, seems 

 to lie in the antero-cardinal region, where the margin of the latter is thickened, 

 causing casts of the interior to appear as broadly impressed in this region. 



Ambonychia planisteiata Hall. 



PLATIS XXXV, PIGS. 3 and i. 



Ambonychia planistriata Hall, 1861. Rep't. Sup't. ^eol. Sur. Wis., p. 32. 



Shell obliquely acuminate-ovate or subrhomboidal, veutricose, wij;h the point of 

 greatest convexity near the center of the antero-cardinal half. Upper half of anterior 

 side somewhat flattened, nearly straight, sloping backward slightly, and more rapidly 

 below, into the basal margin, which, with the greater part of the posterior edge, 

 forms a semicircle; postero-cardinal margin subangular, hinge line straight, one- 

 third or a little less shorter than the greatest length of the shell beneath. Beaks 

 prominent, strongly incurved; umbones full and rounded; posterior cardinal slope 

 concave. Surface marked by distinct, broad and shallow concentric undulations 

 and fine radiating striae, of which about twelve occur in 5 mm. at the margin of an 

 average example. These striae, which are flattened and separated by very narrow 

 interspaces, are cancellated by another set of even finer concentric lines. Test very 

 thin, hinge plate narrow, apparently with two cardinal teeth in each valve and no 

 lateral teeth. In good casts of the interior the antero-cardinal lobe is sharply 

 defined. 



This rare and beautiful species is readily distinguished from A. bellistriata Hall, 

 and A. orbicularis Emmons, sp., by its concentrically undulated surface. In this 

 feature it is like Clionychia undata Emmons, sp., but that form, aside from the fact 

 that it has the characters of Clionychia, is less ventricose, of somewhat diff'erent 

 shape and without radiating lines. For comparisons with A. affinis Ulrich, see that 

 species. 



Formation and locality .—From the "Lower Blue limestone" at Mineral Point and Beloit, Wiscon- 

 sin, and the equivalent limestones at Cannon Falls, Minnesota, and Lee county, Illinois. 



Mus Reg. No. 8327. 



