NATURAL SCIENCES OF PHILADELPHIA. 149 



Color. Reddish-gray, the body somewhat clouded with black, 

 the shield paler, clouded, or more usually with a dark baud ou 

 each side above the spiracle, converging in an elliptic form ; a pale 

 dorsal streak. Head uniform pale-brown, tentacles darker. Foot 

 and often the mantle tinged with olive. 



Shell large and thick, the nucleus near posterior left corner ob- 

 tusely rhomboidal, concave beneath. Length about 0.14, breadth 

 0.06 inch. 1 Jaw arcuate, wider near middle, with 20 to 30 riblets 

 denticulating the lower margin. 



Hab. Near south and east sides of San Francisco Bay north- 

 ward to Santa Cruz. It is confined to well wooded hills or damp 

 river-banks, being less able to bear heat tind dryness than our 

 other sluos. I have named it for Dr. C. L. Anderson of the latter 

 place, a zealous naturalist, who has much aided me by collecting 

 this and other species of mollusca. 



This species combines the form and spiracle of Avion with the 

 shell and jaw of Aviolimax, making the generic distinctness of the 

 two forms doubtful, and consequently connecting Avion still more 

 closely with Limax. The minute caudal mucous gland and acute 

 tail of this species also approach the character of Limax, and 

 make the modern separation of these genera into separate families 

 still more unnatural. 



The rule adopted by Agassiz of founding family divisions on 

 modifications of the general forms, and geneva only on those of 

 special organs, will apply well in the case of these animals. But 

 a difficulty arises here in the disagreement of this species in several 

 points from the latest definition of the genus Ay-ion, in those cha- 

 racters above indicated as connecting it with Aviolimax. Still, as 

 Avion is the older genus, it would seem more proper to extend its 

 generic limits to include a more developed shell and jaw than 

 usual, than to make Aviolimax include a species as different from 

 the type in form, position of spiracle, and structure of locomotive 

 disk. The form of the caudal termination seems like that of 

 Avion hovtensis. Orifice of generative organs about half-way be- 

 tween tentacle and shield as in Limax, etc. 



In colors, this species is so similar to " A. foliolaius," Gld., that 



1 The shell represented in the figures is from a larger specimen than the^ 

 animal figure. 



