334 Report of the State Geologist. 



LindstrcEmella aspidium. 



Plate 3, figs. 1-4. 



Lindstroemella aspidium, Hall. Palaeontology of N. Y., vol. VIII, pt. 1, 

 p., 178, pi. iv e, figs. 25-28. 1892. 



This species is readily distinguished from Orhiculoidea {Bcemer- 

 ella) grandis, its associate in the fauna of the Hamilton shales, 

 and the only form with which there is danger of confounding it, 

 by the convex pedicle-valve, the distant, elevated, concentric 

 surface-ridges, which have a peculiar undulation as they approach 

 the margins of the foramen. The internal characters of the 

 shell are at once distinctive. (See discussion of these features 

 on page 184, op. cit.) Length and width of a mature specimen, 

 50 mm. 



Hamilton group. Leonardsville, Hamilton^ Darien and Coman- 

 daigua Lake^ N. Y. 



Schizocrania Schucherti. 



Plate 3, figs. 5-7. 



Schizocrania Schucherti, Hall. Palaeontology of N. Y., vol. VIII, pt. 1, p. 179, 

 pi. iv g, figs. 31-33. 1892. 



Shell small, usually found unattached ; marginal outline sub- 

 ovate. Surface of pedicle-valve flat or slightly concave ; con- 

 centrically striated. Pedicle-aperture broad and sharply triangu- 

 lar. Brachial or upper valve strongly convex, often laterally 

 compressed. Umbo full and rotund, incurved at the apex, which 

 is almost, but not quite, marginal. Surface covered by numerous 

 simple, sharply elevated, uninterrupted striae, frequently crossed 

 by concentric wrinkles. On the interior of this valve only the 

 strong posterior muscular impressions are distinctly shown. 



Hudson group. Cincinnati^ Ohio. 



Schizocrania (?) Helderbergia. 



Plate 3, figs. 8, 9. 

 Schizocrania {?) Helderbei^gia, Hall. Palaeontology of N. Y., vol. VIII, pt. 1 

 p. 179, pi. iv g, figs. 34, 35. 1892. 



Shell subcircular in outline. Upper valve convex ; apex pos- 

 terior and marginal. Surface covered with fine, closely crowded, 

 elevated, radiating lines, which extend to the apex, and increase 

 by intercalation. Lower valve flat and of less diameter than 

 the upper. Apex subcentral, posterior. Foramen apparently a 

 narrow triangular slit extending to the margin. A short median 



