'2'2 KXTKKN.M. SIIKLL. 



/n'ffiiff ill llic cabinet of Mr. Morris weighs I lb., and when the 

 siphunclc is secured it lh>:its with :i half-pound weight in its 

 aperture. The animal would have displaced two pints or :K> Ibs. 

 of water. ;ind. therefore, if it weighed :{ Ihs., the specific -r.-ivity 

 of the animal :in<l shell would scarcely exceed that of salt water. 

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

 the pressure of the water, as to i>-uard against t he collisions to 

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

 Ammonites, whose ovueral form possesses least strength. The 

 Complicated sutures perhaps indicate lobed ovaries; they occur 

 in ii'enera which must, have produced very small eir^s. The pur- 

 pose of the siphuncle (as sii^ii'csT.ed by Mr. Searies Wood) is to 

 maintain the vit ality of t he shell during t lie lonjr life which these 

 animals certainly enjoyed. Mr. Forbes has sn^fsted that the 

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

 Hut. this was not the case with the (')rf hocerata. whose lon^. 

 straight shells were particularly exposed to danger ; in these the 

 preservation of the shell was provided for by the increased 

 and strength of the siphuncle, and its increased vasciilarit v. 



In Kndoceras we find the siphuncle 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 are formed 

 periodically: but if must not be supposed that the shell-musclo 

 e\rr become detached, or that the animal moves the distance of 

 a chamlx r all at once. If is most likely thai the adductors 

 TO\V 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 resls. 

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

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

 sufficient to convince us that the chambered cephalopoda could 

 not exist in very deep water. They were probably limited to a 

 depth of 'JO or illl fathoms at the ulnio 



The specific gravity <>!' the chambered shells of cephalopods 

 beiii'_f such as to enable themtolloat upon the surface of the 



* The air-chambers would lt> crush, -d by the pressure of water ;it ;my 

 considerable depth ! this pressure exceeilin-j, '.'(;."> Ihs. to the square inch 

 at 100 fathoms at which depth, emptx bottles. sermvl\ corked. 



crushed. 



