264 Systematic Paleontology — Middie Devonian 



Aviculopecten Tpi'inceps Hall, 1884, Pal. N. Y., vol. v, pt. i, Lamellibranchiata 

 i, p. 1, pi. i, figs. 10, 11; pi. V, figs. 18, 19, 23, 24; pi. vi, figs. 1-9; pi. xxiv, 

 fig. 7; pi. Ixxxi, flgs. 13-17. 



Aviculopecten princeps Keyes, 1891, Johns Hopkins Univ. Circ, vol. xi, p. 29. 



Aviculopecten princeps Clarke, 1903, N. Y. State Mus., Bull. 65, p. 365. 



Aviculopecten princeps Grabau and Shimer, 1909, N. Am. Index Fossils, vol. 

 i, p. 487, fig. 650b. 



Description. — Shell large, obliquely broad-ovate; axis inclined more 

 than 60° to the hinge-line; length and height nearly equal; anterior 

 margin convex. Valves depressed; left valve regularly convex; right 

 valve nearly flat, or very moderately convex. Hinge-line straight with a 

 length of from two-thirds to more than three-fourths the length of the 

 shell. Beaks obtuse, rounded, anterior to the middle of the hinge. Ears 

 large ; triangular ; posterior one the larger ; and defined by the abrupt 

 slope of the side of the umbo, while the anterior one is separated by a 

 distinct sulcus; lateral margins concave, becoming convex at the hinge- 

 line. Byssal-sinus broad, rounded, well-defined and indicated on the ear 

 by a sulcus extending to the extremity of the beak. The right valve is 

 flatter and proportionally broader than the left. Test thin, marked by 

 numerous regiilar alternating rays, which increase in number by inter- 

 stitial additions, and become broader and stronger towards the margins. 

 These radiating ribs are crossed by very fine, sharp striae of growth. On 

 the ears the rays are nearly obsolete, and the lines of growth are sharper 

 and stronger than on the body of the shell. The dimensions of the 

 shells of this species are very variable. Large individiials have a height of 

 SO mm. with nearly equal length, and a hinge-line of 50 mm. The grad- 

 ation from this form is very gradual to those in which the height is 

 equal to, or greater than, the length, and where the length of the hinge- 

 line is nearly equal that of the shell. Hall, 1884, condensed. 



The Maryland specimens available for study are broken and quite im- 

 perfectly preserved ; but after comparison with authentic specimens of this 

 species in the New York State Museum it is thought that they are 

 correctly identified. A fragment of a large valve shows nicely the strong 

 radiating rays toward the margin of the shell with smaller intercalated 

 ones. The figured left valve from the Williams Eoad near Cumberland 



