39 MOLLUSCOUS ANIMALS. 



wanting the nacreous lustre, and being apparently an animal feeder, 

 are sufficient in these cases to prevent mistake. 



Among shell characters of most real importance are differences in 

 the form of the mouth, especially the absence or presence of a channel 

 and of appendages, and the mouth being circular or nearly so and com- 

 plete all round, or pressed against the next whorl so as to become lunate, 

 or for the circle to seem broken ; the growth appearing to be uniform 

 or interrupted, a special border being then formed to the mouth at the 

 completion of each period, and the old borders remaining as ornaments 

 on the shell; the absence or presence of tooth-like processes about the 

 mouth, and of folds on the columella, and differences in the substance 

 of the shell itself. Here, however, it must be noted that the channel 

 in the border being accommodated to the opening of the Siphonal canal 

 only indicates its existence and direction. This canal is connected with 

 a burrowing habit, but it is possible for it to be present or absent in 

 animals connected by much more important particulars of structure, so 

 that we cannot implicitly rely upon it in our attempts at natural group- 

 ing. The presence, number, and peculiar form of the Varices or 

 remains of former borders of the aperture may be good generic 

 characters, but could not lead to higher combinations. Folds on the 

 columella are deserving of much attention as auxiliary characters. 

 Tooth-like prejeetions of shelly matter are always of interest, but must 

 be employed with great caution, more especially as our knowledge does 

 not enable us to connect them with any structural peculiarity or special 

 habit of the animal. Monoceros is scarcely now admitted as a genus, 

 and its supposed species hardly even all belong to the same genus. 

 Curious tooth-like projections variously placed around the aperture 

 adorn numerous species of Helicidae and Auriculidae, but their 

 ejstematic value, beyond characterising species, is very doubtful. 



A class of characters much and justly valued since attention has 

 been called to it and offering great assistance in the determination of 

 natural families, is derived from the operculum; its absence or presence, 

 its substance whether shelly or horny, and the mode of its formation 

 whether from a marginal or a central nucleus, as well as its figure. 



A still more important class of characters is derived from an organ 

 characteristic of the higher (the cephalous) Molluscous animals and 

 used by them in obtaining or preparing their food, which has been 

 called their tongue or lingual ribband, but which needs an express 

 name and should be spoken of by that which Huxley has proposed ; 



