69 



KELLIADiE. 



Although there appears to be a limit to the dimension 

 which each species of Mollusk is capable of attaining in the 

 course of its growth, the several species of a genus do not 

 usually exhibit uniformity of limitation, and not unfre- 

 quently we find a very minute and a comparatively gigantic 

 shell in the same group. In the family before us, how- 

 ever, limitation of size to very small dimensions seems to be 

 characteristic of all its members, and thus we are presented 

 with a tribe of minute shells. 



Minuteness of size is often accompanied with eccentricity 

 of variation in specific character, consequently in such a 

 tribe as this the relative value of characters is not easy to 

 ascertain, and the distribution of the species into genera a 

 difficult and critical operation ; the more so, as it is no 

 easy task to observe correctly the soft parts of such small 

 mollusks, whilst the difficulty is increased by the scar- 

 city of many of them in the living state. Indeed, until 

 very lately, we had little, if any, account of the animals 

 of the tribe before us, at least in an accessible form, — 

 yet many years ago the inhabitants of four out of the six 

 genera we have adopted in this work had been carefully 

 examined and delineated by our friend Mr. Clark. To 

 him and to Mr. Alder we owe all our knowledge of them, 

 having ourselves examined the animal of Kellia only. 

 From their manuscripts and some very recently published 



