EXTERNAL SHELL. 23 



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

 being washed :i shore in localities removed many hundreds of 

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

 torily two interesting paheontological facts, namely, the in- 

 numerable 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 and Helicoceras ; these latter instead of form- 

 ing 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, very numerous, vary all the way from a straight to a 

 coiled growth, their difference in plan of growth constituting the 

 generic distinctions; to be enlarged upon in the descriptive por- 

 tion of this work. It may be remarked here, however, that 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 linally grows in a 

 straight line. In Ancyloceras the commencement is similar, but 

 after elongating the whorl for awhile the extremity is incurved. 

 And in Scaphites 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 

 the form but merely the external surface of the shells. Some 

 fossil Nautili, for instance, striated when young, become smooth 

 at a later period, whilst others smooth when young are striated 

 or ribbed when adult. 



So in the Ammonites, the juvenile shell is always smooth, but 

 in the course of growth, tubercles, ribs and stria 1 appear, and 

 develop until the animal has become adult ; after this period 

 degeneration takes place, the ornaments gradually disappear in 

 the successive growths, and in old age the surface of the outer 

 whorl may be as smooth as in youth. In Ammonites of the 

 same species two forms of shells may be observed, one much 

 compressed, the other swollen ; and it is reasonable to conjecture 

 from analogy with living species that the sexes are thus in- 

 dicated to us, the swollen shell being required for the ovary of 

 the female. 



