GASTEROPODA. . n 



the P. ventricosum of the Lower Helderberg group ; but the first volution is 

 more slender, and the spire less closely enrolled, while the lateral pouch-like 

 expansion and the revolving bands are distinctive features. This shell is also 

 much smaller, being rarely more than an inch and a quarter high, with the 

 greatest diameter of the aperture about *equal to the height. The length 

 from the apex to the anterior margin of the aperture is about one inch 

 and a quarter in large individuals. The small specimen (fig. 7) is a very 

 symmetrical form of a young individual of the species which preserves the 

 peristome in contact with the preceding volution. The figure scarcely repre- 

 sents the true character of the specimen. 



Formation and localities. In shales of the Hamilton group, at York and Moscow, 

 in Livingston county, N. Y. 



Plattceras fornicatum. 



PLATE IV, FIGS. 1-8, 7, 8, 18, 19, SO, AND PLATE V, FIGS. 8, 9 [?]. 



Platyceras fornicatum, Hall. Descriptions of New Species of Fossils, etc., p. 7. 1861. 



" '• " Fifteenth Rep. State Cab. Nat. Hist., p. 35. 1862. 



•• - " Illustrations of Devonian Fossils, pi. 4. 1876. 



(?) " - v. amtractum. Hall. Id., pi. 5, figs. 8, 9. 1876. 



" rictum (part), Hall. Id., pi. 4, figs. 18, 19, 20. 1876. 



Shell obliquely subhemispherical, or very depressed, obliquely subconical. 

 Apex minute, distinct, spirally enrolled for about one turn and a half, 

 below which it expands, so that in the extent of an inch and a half along 

 the dorsum to the front, it has acquired an aperture of about an inch and 

 a half in diameter in both directions. The upper side of the spire for 

 the first volution and the following half is flattened ; the angle continu- 

 ing into the broad expansion of the body-whorl, and dying out before 

 reaching the margin, as shown in figs. 1-5. Aperture nearly round or round- 

 ovate ; peristome scarcely sinuous, except at the postero-lateral margin. 



Surface marked by fine concentric stria?, with a few strong spines upon the 

 body-volution. 



