202 Ne'W iSjJCcies of Fuss lb from Ohio. 



Mytilarca percarinata, n. sp. 



Pal. O., Vol. Ill, Plate 6, Figs. 1 and 2. 



Shell less thaa medium size, the specimen used for description and illus- 

 tration measuring but one and three-fourths inches in extreme height ; and 

 the distance from the anterior to the posterior margins across the point of 

 greatest diameter, only a trifle over one inch ; the depth of the valve being 

 nearly half an inch. Form of the shell elongate triangular-ovate, rather 

 acutely pointed at the beak, which is small and incurved ; anterior, or bys- 

 sal, margin straight and absolutely vertical in the example mentioned ; 

 basal margin broadly rounded from the anterior line nearly to the point of 

 greatest length of the valve, where it is more rapidly curved, and finally 

 passes abruptly into the rapidly ascending posterior margin ; the lower 

 part of which is nearly parallel to the anterior side, but above inclines more 

 rapidly toward the short and very oblique hinge-line. The surface of the 

 valve is most elevated along the anterior unibonal ridge, where it is at right 

 angles to the anterioi' surface, but slopes gently backward for two-thirds of 

 the distance toward the pqsterior margin, and on the other third much 

 more abruptly. Near the beak, the surface rounds rapidly from the an- 

 terior ridge to the posterior border. Surface of the shell marked by nume- 

 rous concentric ridges, parallel to the margin of the valve, many of which 

 are strongly marked and form varices of growth. On the anterior surface, 

 these varices and the concentric striae are well marked. Cardinal area not 

 observed. 



Tlic excimp](j used is a riglit valve, and bears evidence in its 

 characters of being an adult shell. It is associated in the same 

 layers of cherty material with M. ponderosa, H. & W. (Prelim. 

 Kotiee Lamell. Shells, etc., p. 21). but may be readily dis- 

 tinguished by the vertical anterior surface and the angular um- 

 boual ridge. From the young of that species, it is readily 

 distinguished by those characters, as those are distinctly round 

 and ventricose. The only known species approaching this in 

 the angularity of the ridge, is M. attenuata, H. & W. , of the 

 Chemung group ; but this is quite distinct in other respects. 



Formation and Locality. — In the white chalky chert-beds of 

 the Upper Helderberg Group, near Dublin, Ohio. 



GASTEEOPODA. 



Platyceras sqiialodens, n. sp. 



Pal. O., Vol. Ill, Plate 3, Figs. 6 and 8. 

 Shell small, sharply conical when viewed in a lateral direction, with the 

 apex gently curved anterior!}^ ; but in a posterior view, the form is narrowly 



