CEPHALOPODA. 387 



Test and surface-markings unknown. Internal mould smooth, with the 

 sutures slightly impressed, and preserving some traces of transverse lines of 

 growth. 



The largest individual observed has a diameter, measured across the disc, 

 of 200 mm., with a diameter of the tube at the aperture of seventy mm. 

 A small, septate fragment, comprising about one-half of one volution, has a 

 diameter across the volutions of 110 mm., and preserves about thirty air- 

 chambers. 



This species is distinguished by its transverse section, its size, closely coiled 

 volutions, and the absence of any prominent ornamentation on the tube. It 

 somewhat resembles G. validum in its smooth internal mould, but the large, 

 closely coiled volutions and transverse section are very different. From G. 

 Cyclops it is easily distinguished by the absence of the annulations and crenula- 

 tions of the internal mould, and is without evidences of any characteristic sur- 

 face ornamentation. 



Formation and locality. In the Magnesian limestone of the age of the Hamil- 

 ton group, near Milwaukee, Wis. 



Gyroceras Cyclops. 



PLATES CI; CII; C1U, FIGS. 1,8; CIV, FIGS. 1,2. 



Gfyroceras Cychips, Hall. Descriptions of New Species of Fossils, etc., p. 40. 1861. 

 Fifteenth Rep. N. Y. State Cab. Nat. Hist., p. 68. 1862. 

 " " " Illustrations of Devonian Fossils: Cephalopoda, pl7 53, figs. 1-3. 1876. 



Shell large, discoidal, regularly coiled. Spiral open, making about one volu- 

 tion and a half. The volutions are distant about twenty mm.; near the 

 aperture they are almost contiguous on account of the expansion of the tube. 

 Transverse section subcircular, or broadly oyal, flattened on the dorsum, and 

 o'utusely subangular on the ventral side. Tube regularly enlarging from 

 the apex. Apical angle about 14°. 



Chamber of habitation small, expanding and forming a campanulate aper- 

 ture, which opens outward, oblique to the spiral axis of the tube. This 



