Maryland Geological Suuvky 645 



the basal margin, which is sometimes nearly straight posteriorly. Post- 

 inferior extremity angular. Posterior margin obliquely truncate. Car- 

 dinal line straight, less than half the length of the shell. Valves de- 

 l^ressed-convex below, becoming gibbous in the middle and above. In its 

 usual condition of preservation the shell is vciy much depressed. Beaks at 

 about the anterior third, prominent. Umbonal slope subangular, defined 

 above, less prominent below. Surface marked by fine concentric strige 

 of growth, Avhich arc pai'tially picserved in the cast. Three specimens 

 measure respectively 33, 35, and 43 mm. in length, and 25, 29, and 30 

 mm. in height. This species resembles S. appressus in form and pro- 

 portions and is probably only a variety of that species which lived under 

 different conditions." Hall, 1885. 



The shells referred to this species have a more gibbous umbo and are 

 more convex than the specimens figured by Hall appear to be. They are 

 also somewhat smaller in size. They difi'er from S. oherni in being larger, 

 more transverse, more obliquely truncated behind, umbonal slope more 

 angular. 



Occurrence. — Jennings Formation, Chemung Member. Town 

 Creek, 3496 cf., 3453 abundant, 4631? 



Collection.- — Maryland Geological Survey. 



SCHIZODUS CHEMUNGENSIS VAR. QUADRANGULARIS Hall 



Plate LXIV, Fig. 6 



Cytlierodon (Schizodus) quadrangularis Hall, 1870, Preliminary Notice 



Lamellibr. Shells, pt. ii, p. 96. 

 Schizodus Chemung ensis var. quadranglaris Hall, 1884, Pal. of N. Y., vol. v, 



pt. i, p. 454, pi. Ixxv, figs. 31-34, 3G. 

 Schizodus quadrangularis Gralaau and Shimer, 1909, N. Amer. Index Foss., 



vol. i, p. 482, fig. 642. 



Description. — Shell subrhomboidal, with angles rounded ; beaks sub- 

 central; anterior margin semicircular, posterior sloping and subtruncate; 

 postlateral angle due to prominence of umbonal ridge, conspicuous but 

 not acute; posterior slope from the umbonal ridge concave; umbones 

 concave and sloping regularly forward and downward. Surface smooth. 



