'2*2 EXTERNAL SHELL. 



/n'l i it* in the cabinet of Mr. Morris weighs 1 lb., and when the 

 siphuncle is secured it floats with a half-pound weight in its 

 aperture. The animal would have displaced two pints or 2-5 Ibs. 

 of water, and, therefore, if it Aveighed 3 Ibs., the specific gravity 

 of the animal and shell would scarcely exceed that of salt water. 

 The object of the numerous partitions is not so much to sustain 

 the pressure of the water, as to guard against the collisions to 

 which the shell is exposed. They arc most complicated in the 

 Ammonites, whoso general form possesses least strength. The 

 complicated sutures perhaps indicate lobed ovaries; they occur 

 in genera which must have produced very small eggs. The pur- 

 pose of the siphunolo (as suggest-cd by Mr. Searles Wood) is to 

 maintain the vitality of the shell during the long life Avhich these 

 animals certainly enjoyed. M r. Forbes lias suggested that the 

 inner course of Hamites broke oil' as the outer ones were formed. 

 But this was not the case Avith the Orthocerata, whose long, 

 straight shells were particularly exposed to danger; in these the 

 preservation of the shell AVMS provided for by the increased size 

 and strength of the siphuncle, and its increased vasculavity. 



In Kndoeeras wo find the siphuucle thickened by internal 

 deposits, until in some of the very cylindrical species it forms an 

 almost solid axis. It has been stated that the septa arc formed 

 periodically; but it must not be supposed that the shell-muscles 

 I'ver become detached, or that the animal moves the distance of 

 a chamber all at once. It is most likely that the adductors 

 grow only in front, and that a constant waste takes place 

 behind, so that they are always moving forward, except when a 

 new septum is to be formed ; the septa, indicate periodic rests. 

 The consideration of this fact, that the Nautilus must so fre- 

 quently have an air-cavity between it and its shell, is alone 

 sufficient to convince us that the chambered cephalopods could 

 not exist in very deep water. They were probablv limited t<> a 

 depth of 20 or :',( fathoms at the utmost.* 



The specific gravity of the chambered shells of eophalopods 

 being such as to enable them to float, upon the surface of the 



* The ail>chamber8 would be, crushed by the pressure of \v;iter ;U ativ 

 considerable depth : this pressure exceedmt;- '_Hir> Ibs. jo The s<|u:irr inch 

 at 100 fathoms -at which depth, empty Unities, securely corked, :ire 



crushed. 



