CEPHALOPODA. 251 



Orthoceras tetricum. 



PLATES LXXVin B, FIG. 4 ; L.XXX, FIGS. 2, 5, 6, 8, 9. 

 Orthoceras tetricum, Hall (see 0. pravum). Descriptions of New Species of Fossils, etc., p. 45. 1861. 



Shell straight, regularly and very gradually enlarging from the apex. Trans- 

 verse section circular. Apical angle varying from 4° to 5^°. Initial 

 extremity unknown. 



A small portion of the chamber of habitation has been observed, having a 

 length of fifty mm. This fragment shows nothing more than a regular 

 expansion conforming with the general contour of the tube. Air-chambers 

 large, extremely irregular in depth, varying from twelve to twenty-two 

 mm. Aperture unknown. 



Septa thin, smooth, very concave. The concavity is equal to an arc of 

 about 140°. Sutures straight, and horizontal. 



Siphuncle excentric, situated a little less than one-third the diameter of 

 the tube from the ventral (?) side. Its elements, in its passage through the 

 chambers, have not been observed ; at the septa the diameter is nearly four 

 mm., where the entire tube has a diameter of thirty mm. 



The test was probably very thin, as it has only been observed in two indi- 

 viduals. Its thickness, as imperfectly preserved, is not more than one mm. 

 Surface marked by very fine, sharp, regular, gently-curving, concentric striae, 

 as shown in fig. 4 of pi. 78 B. 



The internal mould shows the walls of the chambers to be smooth, and 

 continuous with the regular enlargement of the tube. 



No entire individuals have been observed. The longest fragment has a 

 length of 130 mm., with a diameter at the larger end of forty mm., measur- 

 ing twenty-eight mm. in diameter at the smaller extremity. The entire 

 specimen was probably several times the length of this fragment. Several 

 fragments of larger individuals have a diameter of about fifty mm. 



This species is easily distinguished from 0. procerus by its more gradually 

 enlarging tube, and large, deep air-chambers ; and also that none of the specimens 



