GASTEROPODA. 25 



Platyostoma euomphaloides. 



PLATE X, FIGS. 27-29. 

 Platyostoma euomphaloides. Hall. Illustrations of Devonian Fossils : Gasteropoda, pi. 9. 1876. 



Shell depressed subhemispheric ; spire moderately elevated ; volutions three 

 or four, gradually enlarging from the apex, — the last one expanded, 

 not ventricose, with its exterior or periphery flat or slightly convex, 

 and flattened or gently concave in the middle above ; suture-line 

 close. Aperture expanded, broadly subovoid ; the inner lip has a 

 thickened callus, which projects in a prominent rim along the posterior 

 border of the aperture. 



Surface marked by fine concentric striae, which are crowded in fascicles. 



At present, we know only a single specimen having the character described. 

 The flattened upper side of the volutions and the flattened periphery are very 

 unlike any form of Platyostoma described, but at the same time it is not difficult 

 to see how a still farther degree of variation in P. lineata var. callosa may 

 result in such a form as this. In that variety, the summit is depressed, but 

 the volutions are rotund and rapidly expanding, with an aperture suborbic- 

 ular. Unlike the prevailing or characteristic forms of P. lineata, that one has a 

 thickened callosity on the inner lip, but less defined and developed than in the 

 specimen described as P. euompholoides. 



Platyostoma Strophics. 



■instmiia Strophius, Hall. Descriptions of New Speeies of Fossils, etc., p. 13. 1861. 

 " " Fifteenth Rep. N. Y. State Cab. Nat. Hist., p. 41. 1862. 



fl mtr.T, obliquely ovoid. Apex minute; spire ascending, composed of about 

 three or four volutions, very gradually expanding above ; the body- 

 whorl ventricose, somewhat depressed-convex on the side, and sloping 

 downward rather than swelling out in a rotund form ; the upper volutions 

 Standing out prominently above the last one. Suture-line narrow, even, 

 4 



