304 Systematic Paleontology 



" Internally the ventral valve is marked by a large foliate vascular im- 

 pression : impressions of the adductor muscles rarely well preserved, except 

 in the casts. Teeth prominent, and, when entire, rounded and thickened 

 at their extremities. Dorsal valve with prominent cardinal process and 

 divergent brachial lamellae : a central ridge, more or less prominent, ex- 

 tends from beneath the beak to near the base." Hall, 1859. 



Length of average specimen about 2 cm. ; width about 2.5 cm. 



This well-known and widely distributed Helderberg orthoid is abun- 

 dantly present in Maryland, where it rarely attains the large size of 

 the New York examples, but otherwise is identical. 



Occurrence. — Helderberg Formation, Coeymans Member. Dawson. 

 NEVf Scotland Member. 21st Bridge (shale zone), Dawson, Miller's 

 Spring, Devil's Backbone, Corriganville, near Cumberland, Ernstville, 

 Maryland; Cherry Eun, West Virginia. 



Collection. — Maryland Geological Survey. 



Ehipidomella assimilis (Hall) 

 Plate LV, Figs. 17-19 



OrtMs assimilis Hall, 1859, Nat. Hist. N. Y., Pal., vol. ill, p. 175, pi. xv, figs. 



la-le, 1861. 

 Rhipidomella assimilis Hall and Clarke, 1892, ibidem, vol. viii, pt. i, p. 224. 



Description. — " Shell suborbicular, sometimes a little longer than wide : 

 valves nearly equally convex above the middle. Dorsal valve most convex 

 in the middle, and sloping to the front and sides. A-'entral valve most 

 convex towards the beak, depressed and broadly sinuate below : beak 

 prominent, acute, incurved, and extending beyond the opposite valve. 

 Area longer than half the width of the shell ; the height equal to one-third 

 the length. Foramen large. 



" Surface finely and somewhat evenly striated. Vascular impressions 

 of the ventral valve foliate, occupying a broad ovate space, limited on the 

 sides by a continuation of the brachial lamellae." Hall, 1859. 



Length about 2.7 cm. ; width about 3 cm. 



With R. ohlala occur individuals less oblate and more elongate. These 

 approach R. assimilis and it appears that the latter developed from that 



