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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 

 " guard " 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 striae 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, 



