Dr. J. E. Gray on the genus Assiminia. 



425 



observations, appears to regard the shell as of minor importance, 

 the operculum as of still less, and the dentition, because it offers 

 many forms, as of none, in a generic point of view. 



I believe these notions to have arisen from these authors not 

 having had sufficient opportunities of studpng the animals with 

 the shells, or the operculum and teeth with them both, and of 

 seeing how thoroughly they depend on each other, and what 

 exceUent and permanent characters they afford, both separately 

 and in combination with each other. 



For these reasons, although the ancient authority of Dr. Phi- 

 lippi or even any more modern authority may be quoted against 

 me, I must persist that I cannot believe animals such as Trun- 

 catella and Assiminia should be placed together in the same 

 group, for the following reasons : — 



Truncatella 



1 . Has a very short foot and 

 large lips, and walks like a 

 " looping " caterpillar. 



2. Has large eyes furnished 

 with a broad white iris, sunk 

 into the superior and nearly 

 terminal point of short rectan- 

 gular tentacula. 



3. Has according to Dr. Phi- 

 lippi a " non-spiral " opercu- 

 lum. 



4. Has an animal which, in 

 its young state, forms a very 

 slender elongated shell, then 

 suddenly enlarges its diameter, 

 and when it has increased its 

 larger shell to a certain size, 

 suddenly withdraws its body 

 from the upper more slender 

 part, forms a septum across the 

 cavity behind it, and throws off 

 the slender tip, leaving a trun- 

 cated end. 



5. The Truncatell(£ live in 

 the sea. 



Assiminia 



1. Has moderate-sized lips 

 and a large foot, and walks like 

 a Littorina. 



2. Has moderate-sized black 

 eyes on the top of short, nearly 

 cylindrical tentacula. 



3. Has a distinctly spiral 

 operculum of few whorls. 



4. Has an animal which forms 

 a conical shell, which gradually 

 increases in even proportion 

 from its birth to its death, and 

 always retains the shell of its 

 young state on the tip of the 

 older one. 



5. The Assiminia live in 

 rather brackish water often far 

 from the sea. 



Let it be understood, that these are peculiarities which do not 

 belong to a single species of each genus, as Mr. Clark appears 

 to believe, but to each of the several species which form these 



