356 
say, is the precise homologue, in position and in structure, of the 
guard of the Belemnite. 
Barrande endeavored to show this plug to have been secreted by 
external organs, as he supposed—two arms stretching back from 
the aperture like those of Argonauta, and reaching beyond the 
broken apex. The dorsal fold of Nautilus is, however, a secreting 
organ stretching back over the shell; and, as the probable homo- 
logue of the plug-secreting organ of the Orthoceratites and the 
guard-building organ of the Belemnoidea, it enables us at once to 
explain how the Belemnoidea arose from the Orthoceratites, and 
why Aulacoceras had an imperfect mantle. This fold, which was 
far larger among the ancient Orthoceratites, would have been 
necessarily open on the ventral side, then more but not completely 
closed in Aulacoceras, and finally completely closed in the later 
Belemnoidea, and able to construct a guard as perfect as that which 
they carry. 
The solid guard of these animals, a compact cylindrical body 
such as they were known to possess, could have been only a heavy 
burden to a swimming animal. The Belemnoidea, therefore, were 
not purely natatory ; but for these and other reasons, which we 
cannot here discuss, they were evidently ground-swimmers, prob- 
ably boring into the mud for shelter, or as a means of concealing 
themselves while lying in wait for their prey. 
The old view, that the guard could have been in any sense a 
‘‘ouard ’’ against collisions with rocks, etc., in their wild leaps 
backwards, is inadmissible for many reasons. The most obvious 
are its position as an internal organ, its solid structure, and its 
weight. I think it more reasonable to suppose that it might 
have increased the liability to injury from collisions. In tracing 
the Belemnoidea to the Orthoceratites I have simply continued 
the labors and carried out more fully the sagacious inferences of 
Quenstedt and Von Ihering. 
The modern Sepioidea are known to be almost exclusively swim- 
mers; and the more ancient, normal, flattened forms, and their 
descendants, the cuttle fishes, have very light, flattened, internal 
shells, in which the striz of growth are remarkable for their for- 
ward inflection on the dorsal aspect, due to the immense compara- 
tive length of this side of the aperture. 
The enclosure and suppression of the shell was predicted, with a 
“sagacity which commands our highest admiration, by Lankester, 
