CEPHALOPODA. 287 
Test thin, and in the specimen described, remaining imbedded in the sur¬ 
rounding rock. Surface ornamentation unknown. 
The internal mould presents a very smooth appearance, and is marked by 
the ventral carina with traces of fine longitudinal striae. 
The specimen has a length of nearly sixty mm., and comprises about thirty 
air-chambers. 
This species somewhat resembles the apical portion of O. Marcellense, with the 
ventral carina on the internal mould ; but the position and size of the siphuncle 
are distinctive, and its geological position is very different. 
Formation and locality. In the coarse shales of the Hamilton, belonging to 
the upper members of the group, in the central portion of the State. 
Orthoceras spissum, n. sp. 
PLATE LXXXV, FIGS. 6-9. 
Shell straight, regularly enlarging. Transverse section very broadly elliptical. 
Apical angle about 6^°. Initial extremity unknown. 
Only a small fragment of the chamber of habitation has been observed, 
which possesses no distinctive features. Air-chambers very numerous, and 
gradually increasing in frequency toward the apex, having a depth of from 
two to three mm., in the length of thirty mm., where the tube has a diameter 
of about fifteen mm. Ventral side indicated in the cast by an obscure, 
flat carina, visible only under a lens. 
Septa smooth, thin, slightly flattened in the centre, with a concavity equal 
to an arc of about 67°. Sutures straight and horizontal. 
Siphuncle small, nearer to the dorsal side, having a diameter of one mm. 
at the septa, where the tube has a diameter of fifteen mm. Its elements 
have not been observed in the interseptal spaces. 
The test has a thickness of five mm., but the surface characters are not 
preserved in the specimens. 
The internal mould is marked, in a well-preserved fragment, by indistinct, 
