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which however was a small part only of the whole length of the 

 cone. This is the simplest form: and others are, the bent or 

 arcuate, cyrtoceratitic ; the loosely coiled, but with whorls not in 

 contact, gyroceratitic ; the closely coiled, with whorls in contact, 

 nautilian ; and the still more closely coiled or involute shells, the 

 involute nautilian, in which the outer whorls may simply overlap 

 the inner, or entirely conceal them by their excessive growth, as in 

 Nautilus pompilius. 



The Ammonoidea in the earlier forms, the Goniatitinse of the Silu- 

 rian,* had apertures with well-marked ambulatory sinuses sufficient to 

 show that they must have had considerable powers of rising or leap- 

 ing in the water, if not swimming, like the Nautilus. In the later 

 forms of the same suborder and in the Ceratitinse, Ammonitinae and 

 Lytoceratinae the ambulatory sinus is absent ; and in its place a 

 projecting crest or rostrum was developed indicating reduction in 

 size and disuse of the hyponome. This and the generally open 

 apertures enable us to see that they were more exclusively bottom- 

 crawlers than the Nautiloidea. 



The most interesting of the facts in this order lies among the 

 exceptional shells, some of which must have been sedentary, and 

 could neither have crawled nor moved about with any ease ; but none 

 of these, so far as we know, seems to have exhibited a type of aper- 

 ture which indicated transition to an exclusively swimming habit. 

 These shells appear in our subsequent remarks among phylogerontic 

 and pathologic types. 



The Belemnoidea of the Jura had a solid cylindrical body, called 

 the guard, attached to the cone -like internal shell, and partly 

 enclosing it. iVulacoceras of the Trias, as described by Branco, is 

 a transitional form with an imperfect guard, which frequently con- 

 tains fragments of other shells and foreign matter. This demon- 

 strates that this guard could only have been built by some external 

 flap or inclosing sac, independent of the true mantle. This false 

 mantle must have inclosed both the shell and the guard, and must 

 have been at the same time open, so as to admit the foreign mate- 

 rials which Branco found built into the substance of the guard. 

 One of the straight shells of the Silurian Nautiloidea, Orthocera- 

 tites truncates, regularly breaks off the cone of its shell, and then 

 mends the mutilated apex with a plug. This plug, we are able to 



* See Plate ii. 



