244 



PICTURESQUE SKETCHES 



tilus, the generic distinction being founded partly on 

 the position of a siphon or tube which connects one 

 chamber with another, partly on the shape of the 

 mantle. 



It is easy to imagine, however, that in thus de- 

 i. 103 veloping itself round 



an axis, the successive 

 whorls may or may not 

 actually touch one an- 

 other. The common 

 Spirula is an example 

 of the latter structure 

 in a recent shell ; and 

 the shell called Crio- 

 ceratite* (103) corre- 

 sponds with the ammo- 

 nite, much as the spirula corresponds with the nautilus. 

 Again, if, instead of being thus formed continu- 

 ously in a spiral, the animal at particular periods of 

 its growth went off in a straight form, and then again 

 curved, we should have a hooked shell resembling 

 that figured in the annexed cut, and called a Ha- 



Fig. 104 



CRIOCERATITE. 



HAMITE. 



mite\ (104), or another having a more boat-like form, 

 and called by naturalists Scaphite.^ But if, instead of 



* Kpiof (cn'os), a battering-ram ; 

 f 'A/LIJJ (/me), a reaping-hook. 



ceras\ a horn. 

 % SiccHpoQ (scaphos), a boat. 



