CEPHALOPODA. 
305 
tions which have been made from this group, and without especial reference to 
any particular class of fossils, it is very probable that future collections will 
furnish a larger number of Orthocerata. The species here described seem to 
possess few characters in common with the forms of this genus in other groups. 
Orthoceras Atreus, n. sp. 
PLATES LXXXVIII, FIG. 1; LXXXIX, FIGS. 10, 11. 
Shell large, straight, robust, very regularly and gradually enlarging to the 
chamber of habitation. Transverse section circular. Apical angle about 6°. 
Initial extremity unknown. 
Chamber of habitation subcylindrical, well developed; length about three 
times the diameter of the tube at the last septum. Tube with a very broad 
and gentle constriction anterior to the middle of the outer chamber, and an 
indication of a contraction at the aperture. 
Air-chambers regular, deep, having a depth of fourteen mm. where the 
tube has a diameter of fifty-five mm. The external walls are smooth in the 
cast, with a slight longitudinal carina indicating the ventral side. 
Septa smooth, so far as observed, with a concavity equal to an arc of 120°. 
Sutures straight and horizontal. 
The siphuncle, as determined from a small fragment referred to this spe¬ 
cies with some doubt, is large and excentric, having a diameter at the septa 
of five mm., where the tube has a diameter of fifty mm., and distant from 
the nearest margin of the septa about one-third the diameter of the tube. 
It is expanded in the interseptal spaces, as indicated in a longitudinal section. 
The test is not preserved. The internal mould shows evidences of a trans¬ 
versely lamellose-striate surface. 
A fragment, embracing a large portion of the chamber of habitation and 
nine air-chambers, has a length of 300 mm., of which 160 mm. pertain to 
the chamber of habitation. A smaller fragment, referred to this sjiecies and 
preserving its normal form and proportions, contains nine air-chambers and 
has a length of 115 mm. 
39 
