36 THE SHELL. 



Woodward says (Man. Moll., p. 184) : " The use of the air- 

 chamhers is to render the -whole animal (and shell) of nearl}^ the 

 same specific gravity with the water. Thus a Nautilus Pompiliiis 

 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 lbs. of water, and, 

 therefore, if it weighed 3 lbs., 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 are most complicated in the Ammonites, 

 whose general form possesses least strength. The complicated 

 sutures perhaps indicate lobed ovaries ; they occur in genera 

 which must have produced very small eggs. The purpose of the 

 siphuncle (as suggested hy Mr. Searles Wood) is to maintain the 

 vitality of the shell during the long life which these animals 

 certainly enjoyed. Mr. Forbes has suggested that the inner 

 course of Hamites broke off as the outer ones were formed. But 

 this was not the case with the Orthocerata, whose long, straight 

 shells were particularly exposed to danger ; in these the preserva- 

 tion of the shell was provided for by the increased size and 

 strength of the siphuncle, and its increased vascularity." 



The specific gravity of the chambered shells of cephalopods 

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

 water, explains the cause of large quantities of shells of Spirula 

 being washed ashore in localities removed many hundreds of 

 miles from the habitat of the animal ; it also explains satis- 

 factorily two interesting palseontological facts, namely, the 

 innumerable quantity of fossil shells found in beds which represent 

 ancient beaches, and their absence from those beds which formed 

 sea-bottoms. 



External cephalopodous shells are all symmetrical except the 

 genera Turrilites (xxxiii, 3*7), and Helicoceras; these latter 

 instead of forming a spiral rolled in the same plane, are obliquely 

 Spiral ; that is, on one side is the projecting spire of the shell, on 

 the other the umbilical opening or axis of the volutions. The 

 symmetrical forms, ver}- numerous, vary all the way from a 

 straight to a coiled growth, their difference in plan of growth 

 constituting the generic distinctions. In some genera a change 

 of form takes place after they have attained a certain age. In 

 Lituites the shell commences with an open spiral ( with disjointed 

 whorls), and finally grows in a straight line. In Anc3doceras 

 (xxxii, 33), the commencement is similar, but after elongating 

 the whorl for awhile the extremity is incurved. And in Scaphites 

 (xxxii, 35), a similar mode of growth to Ancyloceras is 

 distinguished nevertheless by the initial spiral whorls being in 

 contact. All other modifications caused by age, do not change 



