540 THE MICROSCOPE. 



The relations of the freshwater operculata are as varied 

 as those of the land. Ampullaria seems to find its nearest 

 marine relative in Natica, an opinion which seems con- 

 firmed by the form of the shell. A West Indian species 

 found on the trees of the forests of those islands, and placed 

 by Lam arch in the ffelicina, would rather appear to belong 

 to Neritina. The several peculiarities of their teeth, espe- 

 cially that of H. nemoralis, with its numerous uncini, its 

 sub-opaque trapezoid laterals, which seem heretofore to 

 have been overlooked, confirm the belief in its close rela- 

 tionship to Neritina. The horny mandibles of the Mol- 

 lusca may be deserving of some attention with a view to 

 the elucidation of their affinities. In Cydotus translucidus 

 the mandible is divided into two portions by a median 

 articulation, and it is covered with fine denticulations 

 in regular rows, somewhat like that of Velutina, Plate Y. 

 No. 109. In most of the inoperculata, the mandible is 

 horse-shoe-shaped, and striate or corrugate. In Ampul- 

 laria, the same organ is beak-shaped, like the upper man- 

 dible of Octopus or Loligo. 



11 The lingual band, we should premise, has been, for con- 

 venience of description, divided into longitudinal areas, 

 which are crossed by many rows of teeth. There are 

 five, distinguishable by the different characters of the teeth 

 they bear ; but the characteristics are not always present. 

 The teeth are consequently named median, lateral, and 

 uncini, although the latter are not necessarily more hooked 

 than the others. The areas bearing the uncini have been 

 called pleurae. Since each row is a repetition of all the 

 rest, the system of teeth admits of easy representation by 

 a numerical formula, in which, when the uncini are very 

 numerous, they are indicated by the sign oo (infinity), and 

 the others by the proper figure. Thus, oo 5 * 1 5 oo , 

 which represents the system in the genus Trochus, signifies 

 that each row consists of one median, flanked on both 

 sides by five lateral teeth, and these again by a large 

 number of uncini. When only three areas are found, the 

 outer ones are to be considered as the pleura 3 ,, inasmuch as 

 there is frequently a manifest division in the membrane 

 between them and the lateral areas." 



Most of the Cephalopod molluscs are provided with 



