CEPHALOPODA. 
251 
Orthoceras tetricum. 
PLATES LXXVIII B, FIG. 4 ; LXXX, 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 strife, 
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 
