488 Transactions.—(G eology. 
rounded and perforate. Alveolar cavity one-fourth the total length of the 
guard. 
Var. y.—Lateral furrows long and deep, and curved to the dorsal aspect. 
The length of the guard is five times that of the fifteen septa in the phrag- 
macone. The form of the guard is hastate, owing to the expansion of the 
alveolar walls and the tapering of the apex. 
Var. §.—Guard sub-cylindrieal, and constricted at the alveolus. Lateral 
furrows short, and suddenly reflected to the dorsal aspect. Apex blunt, 
with a terminal tubercle or knob. Length of alveolus one-fourth that of 
guard. : 
Var. «.—Lateral furrows short, deep, and bent towards the ventral 
aspect. Guard depressed on both dorsal and ventral surfaces. Apex sub- 
conical. Angle of the phragmacone 25°. Septa very close, numbering 
25-90 
The series of sections exhibited illustrate the structure of this belemnite, 
and especially display the mode in which the central portion of the guard 
frequently exfoliates, casting from its interior a smooth fusiform body, 
produced and slightly laminated in structure at the upper end, and blunt 
with a minute depression or perforation at the lower end. These form the 
Acanthocomax of Miller, and have frequently been mistaken for spines of 
Cedaris. "They are not formed, as has been suggested, by the abrasion or 
weathering of the guard of the belemnite, but are due to its structural 
arrangement, which, notwithstanding the appearance of radiating prisms in 
the interior, really consists of concentric lamins arranged in fusiform layers 
round the central axis. In some of the specimens the axis is seen to be an 
open canal filled with the sand of the imbedding matrix. This canal seems 
to be continuous with the siphunele that traverses the septa of the phrag- 
macone, and to perforate or be a continuation of the conotheca, passing on 
one side of the spherical pellet or nucleus that forms the apex of the 
phragmacone. 
None of the varieties into which I have sub-divided Belemnites australis 
can be considered to have a peculiar horizon or stratigraphical distribution. 
The lower greensand in the Amuri section has been divided as follows in 
descending order :— 
1. Black grit or car-stone. 
2. Aporrhais ornata beds. 
- 9. Trigonia sulcata beds. 
4. Belemnite beds. 
5. Caleareous conglomerate. 
6. Wood sands, 
