542 



LECTURE XXII. 



200 



obtuse or right angle to the last-formed margin of the shell, and after 

 having formed a calcareous plate in this position, 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 {Scalar ia pretiosa, fig. 200) are due. Should the 



margin of the mantle, instead of being uni- 

 formly 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 soldered to the margin of the shell. 

 This development of pallial calcifying 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 periodical ex- 

 cretion of the excess of calcareous matter in the 

 blood is greatest in the carnivorous univalves. 



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 Dentalium, or of more ordinary proportions, as in 

 the Limpets {Patella). The apex of the cone is always oblique and 

 excentric--, directed in the Limpets towards the head, but in other 

 Gastropods towards the opposite extremity of the body. The 

 conical univalve shell is generally spirally convoluted, sometimes in 

 the same plane, e.g. Planorbis, but more usually in an oblique 

 direction. The apex of the shell (a) is formed by the nucleus, or 

 part developed in the e^^: it is mammillated in Fusus ayitiguus. 

 The spiral turns of the shell {b b) are called "whirls," the last 

 (b, d), being the " body-whirl." 



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, e.g. Clausilia, Physa, the shell is twisted in the opposite 

 direction, when it is called " reverse" or "sinistral." Some individuals 

 of Bullnus, Parlula, and Pupa, and of a few marine species, as Fusus 



Scalaria pretiosa. 



