CEPHALOPODA. 377 



more closely resembles G. Matheri, but differs in its finer and more regular 

 plications of the expansions, their slighter elevation and apparently more ellip- 

 tical transverse section. 



Formation and locality. In the Upper Helderberg limestone, at Cherry 

 Valley, N. Y. 



Gyroceras Matheri. 



PLATE LV, FIGS. 1-6. 



OfHoemu MstJtoi, Conrad. Ooolaff. Burr. N. Y. : Pal. Dept., Third Ann. Rep., p. 206. 1840. 

 Qf roeew Mnt/uri (Com 1. Ham.. Descriptions nf New Species of Fossils, etc , p. 38. 1861. 

 " " " " Fifteenth Rep. N. Y. State Cab. Nat. Hist., p. 66. 1862. 



" " " Illustrations of Devonian Fossils: Cephalopoda, pi. 55, fijjs. 5, 6, and 



pi. 56, fig. 3. 1876. 



Shell discoidal ; spire open, with the outer portion of the last volution directed 

 outward, tending to become tangent to the spiral. The tube makes about 

 one volution and a half. Transverse section elliptical ; lateral diameter the 

 longer, the two diameters being in the ratio of five to seven. Toward the 

 apex and near the aperture the transverse section is subcircular. Apical 

 angle about 10°. 



Chamber of habitation slightly contracting toward the aperture ; its extent 

 has not been observed. The low, rounded ridges forming the crenulations 

 are continued over the annulations and furrows of the internal mould. 

 Aperture like transverse section, with a sinus corresponding to the sinus of 

 the ornaments. 



Air-chambers and siphunole unknown. Septa smooth, deeply concave. 



Test having a thickness of .75 mm. Surface marked by fine, irregular, 

 lamellose lines of growth, crossed by scattered, indistinct, revolving striae. 

 The ornamentation consists of regular, raised, transverse, foliate, plicate 

 expansions, inclining forward and projecting above the general surface about 

 ten mm. The plications are irregular, and correspond to the frequency of the 

 revolving furrows of the internal mould. The distance between the expan- 

 sions of the test varies considerably on the same and on different specimens. 

 Measured on the ventral side of the outer volution, the distance between 

 48 



