374 PALEONTOLOGY OF NEW YORK. 



The test has a thickness of .75 mm. Surface marked by fine, undulating, 

 lamellose lines of growth. Tube ornamented with numerous, regular, trans- 

 verse, sharply plicated, undulating, lamellose expansions, at right angles to 

 the general surface of the shell. These expansions of the test have an eleva- 

 tion of about two mm., and are distant from each other about four mm., 

 gradually becoming more numerous toward the apex. The sinus forms a 

 distinct furrow along the ventrum, and produces a semicircular, retral bend 

 in the lines of growth and transverse lamellae. The lamellae are not plicated 

 over the sinus. Internal mould essentially smooth. 



The disc, in the specimen figured, has a greatest diameter of 150 mm., and 

 the tube has a diameter of fifty mm. at the aperture. 



This species is more coiled than C. citum, and the lines of growth and trans- 

 verse folds of the test are more abruptly undulating. In G. involve the shell 

 has a greater number of volutions, the internal mould is annulated, and the 

 characters of the test and its ornaments, with the ventral sinus, is distinctive. 

 The specimens of this species usually occur in a very compressed condition, but 

 preserving the test, with its ornaments, and showing the number of volutions. 



Formation and localities. In the Upper Helderberg limestone, at Waterloo and 

 Auburn, N. Y. 



Gyroceras trivolve. 



l'LATES 1,11, KICS. 1-6; LII A, FIGS. 1-7. 



Cyrtorertu trieolvU, Cokrad. Geolog. Surv. N. V. : Pal. Dept., Third Aim. Rep., p. 206. 1840. 

 Oyroctrai trivotvis (Conk.) Hall. Descriptions of New Species of Fossils, etc., p. 37. 1861. 



Fifteenth Rep. N. Y. State Cab. Nat. Hist., p. tin. IMS. 

 " *• " " Illustrations of Devonian Fossils: Cephalopoda, pi. 54, tigs. 1-5 ; and 



pi. 56, fig. 4. 1876. 

 Orthocera paradoxica, Sowbrby. Eaton : Geological Text Book, p. 29. 1832. 



Shell involute, subdiscoidal, openly coiled. Spiral rapidly enlarging, making 

 from one and a half to two volutions. The last half of the outer volution 

 in large individuals shows a tendency to become straight and tangent to 

 the spiral. Transverse section subcircular. Tube regularly and gradually 

 enlarging from the apex. Apical angle about 9°. 



