CEPHALOPODA. 401 



Trochoceras obliquatum. 



PLATES LVIII, FIG. 5 ; CXI, TIGS. 1, 2. 

 Truchoceras oblijuatas. Hall (in part). Illustrations of Devon. Fossils : Cephalopoda, pi. 4S, figs. 5, 6. 1876. 



Shell curved, making about one-half of one volution, which is twisted or bent 

 out of one plane, producing a considerable variation from a symmetrical volu- 

 tion. Transverse section subquadrangular, becoming broadly elliptical or 

 subcircular in some individuals ; lateral diameter the longer. Tube regularly 

 and gradually enlarging from the apex. The enlargement between the lat- 

 eral faces is much more rapid than between the ventral and dorsal sides. 

 Most of the specimens indicate a dextral helix ; but in some, where the suture 

 lines are inclined in a contrary direction, the spiral appears to be sinistral. 



The chamber of habitation has a length equal to its greatest transverse 

 diameter. Some obscure remains of the furrows of the crenulations are pre- 

 served on the cast of the walls of the air-chambers. Aperture unknown. 



Air-chambers increasing in depth to the grand chamber, where they 

 measure six mm. on a specimen in which the diameter of the tube at the 

 point is thirty mm. Septa smooth, moderately concave. Sutures oblique, 

 curving slightly downward over the dorsum, extending toward the aperture 

 over the upper lateral face, and oblique on the ventrum. 



JSiphuncle small, near the ventral side. Test and surface-markings 

 unknown. Internal mould essentially smooth, showing the obscure crenula- 

 tions over the walls of the air-chambers. Sutures slightly impressed. 



One imperfect individual, consisting of the grand chamber, and twenty- 

 five attached air-chambers, has a length of 140 mm., of which forty mm. 

 pertain to the chamber of habitation. A chambered fragment, having a 

 length of 110 mm., preserves twenty-two air-chambers. 



Tli is species is distinguished from, T. Pandion by its more angular transverse 



section, lesser curvature, and more numerous septa. Some of the phases closely 



■inble T. expansum, especially when compressed in a ventro-dorsal direction; 



but that species is more symmetrical, with an elliptical, transverse section, and 



has a nautiloid appearance, which is sometimes slightly imitated by special 



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