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 0. 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 LXXZV, FIGS. 6-8. 



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 , 



