Marylaxd Geological Survey 311 



Occurrence. — Helderbkrg Formation, Coeymans Member. Dawson. 

 Few Scotland Member. Miller's Spring near Cumberland, Devil's 

 Backbone. 



Collection. — Maryland Geological Survey. 



Genus STROPHEODONTA Hall' 



The Lower Devonian species of Stropheodonta are readily divided into 

 two groups on the basis of the ventral muscle areas. In Stropheodonta 

 proper the shells are always concavo-convex and sometimes strongly so, 

 while in Leptostrophia they are nearly always plano-convex (except 

 L. magnivethtra, which is strongly concavo-convex). In the latter group 

 the ventral muscle scars are bounded laterally by two strongly pustulose 

 diverging ridges, while anteriorly the scars are not strongly limited. In 

 Stropheodonta proper there are no lateral diverging ridges and the muscle 

 scars are well defined all around by a raised rim which is more or less 

 scalloped. 



The cardinal areas in all the Devonian forms of Stropheodonta are 

 strongly marked by transverse lines terminating on the hinge-line by a 

 series of fine teeth. In the Silurian species the degree of development of 

 these teeth is, upon the whole, progressive, beginning in the Clinton with 

 a few teeth on each side of the deltidium and increasing in number 

 through the Niagara into the Manlius. For these early forms of Stro- 

 pheodonta Hall and Clarke have used Brachyprion Shaler. 



STROrHEODONTA ARATA (Hall) 



Plate LVII, Figs. 7-10 



Strophodonta varistriata var. arata Hall, 1859, Nat. Hist. N. Y., Pal., vol. iii, 

 p. 183, pi. xviii, figs. la-It, 1861. 



Description. — " Shell semielliptical, with the cardinal extremities more 

 or less salient : hinge crenulate. Dorsal valve more or less concave. Ven- 

 tral valve varying from moderately convex to very gibbous, and sometimes 



^ For an extended description of this genus and its subdivisions, see Hall and 

 Clarke, 1892, Nat. Hist. N. Y., Pal., vol. viii, pt. i, pp. 284-289. 



