162 



TERRESTRIAL AIR-RREATIIING MOLLUSKS. 



Foot not much exceeding in length the diameter of the shell, and terminating 

 in a broad, obtuse, and flat extremity. A light marginal line runs along the 

 edge of the font from the head to the posterior part, those of the two sides 

 meeting in an acute angle. 



Variety: head and neck blackish-brown, eye-peduncles blackish, foot brown- 

 ish, base dirty white. In a single instance the whole animal was entirely black. 

 The animal of the ribbed form of alternata found at University Place, 

 Franklin County, Tennessee, by Bishop Elliott, resembles in length, etc., Cum- 

 berlandiana ; it is dark slate-color on top of head and eye-peduncles; dirty 

 white on bottom of foot; remainder dark orange. 



The variation of color ranges from pale straw to dark 

 reddish-brown, in each extreme being sometimes uniform. 

 In outline the variation ranges from depressed 

 to very globose. In sculpturing it varies greatly. 

 A comparatively smooth variety, with a shin- 

 ing, somewhat translucent epidermis, has been 

 noticed in New York, by Mr. Bland, under the 

 name of var. Fergusoni. A form with stronger 

 striae and well-developed carina is figured in 

 Fie. 71. The coarsely striated form, which 

 I presume to be //. mordax, is figured also 

 (Fig. 72). This is considered by Mr. Bland 

 to be a variety of Cumberlandiana. I have 

 received it from Eastern Tennessee and Vir- 

 ginia. I have also given a figure (Fig. 73) 

 carinated. f t j 10 magnified surface of a strongly ribbed 



form from North Carolina, and a view (Fig. 74) of a strongly ribbed form 

 from the Post-pleiocene. 



Fi ... 73 In New England this is perhaps the most common species of 



the genus. It abounds in the forests, and is not uncommon in 

 the open country in moist situations, where it can find shelter 

 under logs and stumps. It seems to be more gregarious than 

 other species ; at any rate, numbers are more frequently found 

 in the same retreat. It does not bear a change from a moist 

 to a dry situation so well as many other species. In captivity 

 it remains buried a great part of the time under the moist 

 earth, with the body half protruded. If removed to the sur- 

 face, it withdraws within the shell, protects its orifice by three 

 or four coverings, and soon dies unless supplied with moisture. 

 The foot of the animal is smaller and the eye-peduncles 

 shorter than in either of the other species possessing so huge 

 a shell; it is also flatter and thinner. The mantle is deeply tinged with the 

 coloring matter which ornaments the shell, and which is sometimes secreted 



P alternata, 

 var. mordax ? 



Surface of 

 P. alternala. 



