Maryland Gkological Survey 165 



Genus RHIPIDOMELLA Oehlert 



Ehipidomella vanuxemi TIall 

 Plate XIII, Figs. 26-29 



Orthis vanuxemi Hall, 1857, Tenth Rep. N. Y. State Cab. Nat. Hist., p. 135, 



figs. 1-7. 

 Orthis vanuxemi Hall, 1867, Pal. N. Y., vol. iv, pp. 40, 47, pi. v, flg. 6; pi. vi, 



figs. 3a-3r. 

 Orthis vanuxemi Keyes, 1891, Johns Hopkins Univ. Circ, vol. xi, p. 29. 

 Rhipidomella vanuxemi Hall and Clarke, 1892, Pal. N. Y., vol. viii, pt. 1, p. 



225, pi. vi, figs. 14, 15; pi. vi A, figs. 7, 8. 

 Rhipidomella vanuxemi Schuchert, 1897, Bull. U. S. Geol. Surv., No. 87, 



pp. 352, 353. 

 Rhipidomella vanuxemi Clarke, 1903, N. Y. State Mus., Bull. 65, p. 299. 

 Rhipidomella vanuxemi Grabau and Shimer, 1909, N. Am. Index Fossils, 



vol. 1, p. 265, figs. 320a-c. 



Description. — Shell subcircular or transversely suboval, compressed; 

 hinge-line very short; margins of the valves crenulated within from the 

 external striae; interior minutely punctate. Ventral valve nearly flat or 

 a little concave towards the front, moderately convex in the umbonal re- 

 gion; beak small extending little beyond the opposite one; cardinal area 

 very small, less than half the greatest width of the shell; delthyrium 

 comparatively large, triangular and partly filled by cardinal process of 

 opposite valve; the teeth are prominent and the interior of the valve is 

 marked by a large flabelliform didiictor impression, which reaches from 

 one-half to two-thirds the length of the shell and in the median line is the 

 adductor impression. Dorsal valve convex; beak scarcely distinct from 

 the cardinal border; cardinal process prominent, which is continued iir a 

 rounded median ridge for half the length of the shell. Surface marked 

 by fine, closely arranged, radiating tubular striae, which are perforate at 

 intervals and increase both by implantation and bifurcation. These are 

 crossed by fine concentric striae and at greater intervals by concentric, 

 imbricating lines of growth; entire surface when magnified granulate or 

 punctate. 



There are specimens of this shell as well as internal impressions which 

 practically agree almost precisely with specimens from the Hamilton 

 shales of New York. It was also reported by Hall from Maryland and 



