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. 
Cyrtoceras Matheri, Conrad. Geolog. Surv. N. Y.: Pal. Dept-., Third Ann. Rep., p. 206. 1840. 
Gyroceras Matheri (Conr.), Hall. Descriptions of 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, tigs. 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 siphuncle 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 strias. 
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 
