Maryland Geological Survey 249 



Parallelodon hamiltoniae Cleland, 1903, BulL U. S. Geol. Surv., No. 20G, p. G5. 

 Macrodon hamiltoniae Clarke, 1903, N. Y. State Mus., Bull. 65, p. 437. 

 Parallelodon hamiltoniae Grabau and Shlmer, 1909, N. Am. Index Fossils, 

 vol. i, p. 403, figs. 517b, c. 



Description. — " Shell of medium size or larger, subelliptical or sub- 

 ovate, wider behind ; length about twice the height ; basal margin broadly 

 curving, sometimes nearl}' straight in the anterior portion; posterior ex- 

 tremity broadly rounded; sometimes subtruncate in the upper half; 

 anterior end abruptly rounded or subtruncate; cardinal line essentially 

 straight, obtusely subangular at both extremities. Valves convex in the 

 posterior portion and gibbous in the anterior and umbonal portions; 

 beaks subanterior, prominent, rising above the hinge-line. Surface 

 marked by regular, subequidistant, lamellose, concentric lines and by fine 

 radiating striae, which are usually interrupted at the edges of the 

 lamellae and become tliickened at their lower extension; the radii are 

 stronger on the posterior part of the shell." Hall, 1885. 



This species is represented by a considerable number of specimens in 

 the rather coarse Hamilton shales of Maryland and shows all the char- 

 acteristic features. It was also reported by Hall " from the Hamilton 

 group, near Cumberland, Md." (Pal. N. Y., vol. v, pt. i, Lamelli- 

 branchiata ii, p. 350) and a left valve of a large individiial from near 

 Cumberland represented by figure 10 on plate li. The most marked 

 characters of the species are the nearly straight hinge-line with the sub- 

 angular ends; roimded posterior end; strong, distant lamellose concentric 

 lines; and fine interrupted radii, which are strongest on the posterior part 

 of the shell. 



Length, 17-24 mm.; height, 9-13 mm. 



Occurrence. — Eomney FormatiojSt, Hamilton Member. East bank 

 Evitts Creek below Wolfe Mill; on Hancock-Harrisonville Eoad about 2 

 miles north of Hancock ; Ernstville ; along Plintstone Creek in Gilpin. 



Collections. — Maryland Geological Survey; American Museum of 

 Natural History. 



