298 Systematic Paleontology 



Genus DALMANELLA Hall and Clarke' 



DaLMANELLA PLANICONVEXA (Hall) 



Plate LIV, Figs. 1-6 



Orthis planoconvexa Hall, 1859, Nat. Hist. N. Y., Pal., vol. iii, p. 168, pi. xii, 

 flgs. 1-6, 1861. 



Description. — •" Shell plano-convex; outline somewhat circular or trans- 

 versely oval : length and breadth about as ten to twelve. Dorsal valve 

 nearly flat, slightly prominent near the beak on either side of the faint 

 central depression, and quite flat towards the margins. Ventral valve 

 convex, sometimes scarcely subangular towards the beak; greatest con- 

 vexity a little above the middle of the shell, and thence sloping uniformly 

 to the lateral and basal margins : beak small, acute, incurved. Area linear, 

 its length greater than half the width of the shell. Strise fasciculate, much 

 curved upwards towards the cardinal and lateral margins." Hall, 1859. 



In Maryland this species seems to take the place of D. suhcarinata of 

 New York. The latter is very rare, if present at all, in Maryland, and the 

 few dorsal valves seen that may pass as that species are about one-third 

 the size of the jSTew York form. On the other hand D. planiconvexa is 

 common in the South and rare in 'New York. In the Eidgely member of 

 the Oriskany the species is very common and, as a rule, is a little longer 

 on the hinge than New Scotland specimens ; occasionally there is one as 

 extreme as any in the Becraft. Specimens from near Cumberland occur 

 infrequently in which the dorsal valve is depressed convex instead of flat 

 or very slightly concave. In these the cardinal process is thick and very 

 high- 

 Length about 1.6 cm. ; width about 1.5 cm. to 2.5 cm. 

 Occurrence. — Helderberg Formation, New Scotland Member. 

 Corriganville, Miller's Spring, 21st Bridge, Maryland; Keyser, West 

 Virginia. Becraft Member. Cherry Run, West Virginia. Oriskany 

 Formation, Eidgely Member. Knobly, Williams Road near Cumber- 

 land. 



Collections. — Maryland Geological Survey, U. S. National Museum. 



' This group of orthoifls has a punctate shell structure and is readily dis- 

 tinguished from Rhipidomella by the smaller and narrower muscular scars, and 

 the small and inconspicuous cardinal areas. 



