GASTEROPODA. 295 



ment. Its formation in the univalve Gasteropods commences in the 

 embryo, and the first-formed part is called the nucleus of the shell ; 

 the succeeding layers are not, however, formed around this, but are 

 added to the inner surface of the circumference of the previously 

 formed parts ; and the proportions in which the new formed layers 

 extend beyond their predecessors determine the figure of the future 

 shell. In some Gasteropods, at certain seasons, the margin of the 

 mantle in which the shell-forming processes have greatest activity 

 extends outwards at an obtuse or right angle to the last-formed mar- 

 gin of the shell, and after having formed a calcareous plate in this po- 

 sition, the mantle extends in the ordinary direction, increasing the 

 length of the shell, and is again similarly extended at a right angle 

 with the last formed part : it is to this periodical growth of the 

 mantle and plethoric condition of the calcifying vessels that the 

 ridges on the exterior of the shell in the wentletrap (^Scalaria 

 pretiosa) are due. Should the margin of the mantle, instead of 

 being uniformly extended, send outwards a number of detached 

 tentaculiform calcifying processes, these will form a row of spines 

 corresponding in length and thickness to the soft parts on which they 

 are moulded ; and, as the calcification of the processes proceeds, the 

 spines, which were at first hollow, become solidified, and finally sol- 

 dered to the margin of the shell. This development of pallial calci- 

 fying processes or filaments and of the resulting spines, likewise 

 alternates with periods of the ordinary increase of the shell ; and thus 

 its exterior surface may become bristled with rows of spines, as in the 

 Murex crassispina. 



The most simple form of univalve shell is the cone, which may be 

 much depressed, as in the genus Umbrella, or extremely elevated and 

 contracted, as in the Dentalium, or of more ordinary proportions, as 

 in the Limpets. The apex of the cone is always oblique and ex- 

 centric ; directed in the Limpets towards the head, but in other Gas- 

 tropods towards the opposite extremity of the body. The conical 

 univalve shell is generally spirally convoluted, sometimes in the same 

 plane, as the Planorbis, but more usually in an oblique direction. 



As a general rule the spiral univalve, if viewed in the position in 

 which its inhabitant would carry it if it were moving forwards from 

 the observer, is twisted from the apex downwards from left to right, 

 the spire being directed obliquely towards the right ; but in a few 

 genera, as in Clausilia, Physciy the shell is twisted in the opposite 

 direction, when it is called ' perverse ' or ' sinistra.l' Some species 

 of Bulinus, Partula, and Pupa, and a few marine shells, as Fusus 

 sinistrorsus, are sinistral. The part around which the spiral cone is 

 wound is termed the ' columella : ' this is sometimes simple, sometimes 



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