PLEISTOCENE EOSSILS 



131 



Planorbula is similar to Helisoma but smaller, flattened, and 

 the last whorl has several lamellae within the whorl. P. armigera 

 (fig. 381) is common in stream deposits in Ohio. 



Promenetus (fig. 382) is small (1/4 to 3/8 inch), like Planorbula 

 but much flatter, and without internal lamellae. The outer edge of the 

 whorl is pinched in to form a low keel. Several species are common in the 

 Pleistocene of Ohio. 



Gyraulus (fig. 383) is similar to Promenetus in size and shape but it 

 lacks the keel on the outer edge of the whorl. Several species are 

 common in our Pleistocene. 



Fig. 381 



Fig. 382 



The shells of Ferrissia (fig. 384) are unique among freshwater 

 gastropods. They are not coiled, but saucer - 



shaped or cap-shaped. Several species are common in the Pleistocene 

 of Ohio. 



Fig. 385 



Fig. 383 



Fig. 384 The shell of Physa (fig. 385) is similar to that of Lymnaea but 



coils in the opposite direction. Compare figures of the two genera and 

 note the position of the aperture. Physa can be confused only with Aplexa which 

 is the only other shell that coils in the same direction. Several species of Phys a 

 are found in both lake and river deposits. 



Aplexa (fig. 386) is similar to Phys a but the spire is narrower 

 and more slender. Moreover, the shell of Aplexa has a metallic sheen 

 which is preserved in fossil shells. A., hypnorum is common in pond 

 deposits. 



b) Land Genera 



Fig. 386 



Fig. 387 



Anguispira (fig. 387) has a depressed shell, wider than high, and 

 a wide umbilicus. The surface is covered with irregular brown mark- 

 ings, sometimes preserved in fossil shells. The aperture is not 

 reflected. A., alter nata is one of the commonest land snails of the 

 Ohio Pleistocene and its shells are found commonly in lake and river 

 deposits. 



Mesodon (fig. 388) has a large shell, with a moderately high 

 spire; the body whorl is large, evenly rounded; lip reflected all 

 round, the inner margin covering the umbilicus. In some species 

 the aperture has internal lamellae which may be so large as to 

 fill the aperture almost completely. Several genera ( Allogona , 

 Triodopsis , Stenotrem a) resemble Mesodo n so closely that they 

 are not separated from it here. These land snails are common in 

 lake deposits into which they were washed from the surrounding 

 land areas. They are called Polygyra or Helix in older publications 



Fig. 388 



The shell of Succinea (fig. 389) resembles that of Pseudosuccinea very 

 closely, but it may be distinguished from it by the presence of axial striations 

 only on the surface; Pseudosuccinea has both axial and spiral striations. Three 

 species of Succinea and the closely related genus Qxylom a are common in lake 

 deposits. 



Fig. 389 



The shell of Haplotrema (fig. 390) is similar to that of 

 Anguispira ; it has the same wide umbilicus and simple lip, but 

 it lacks the color markings of Anguispir a. The whorls increase 

 in size more slowly than in Anguispira . H. concavum is abun- 

 dant in some of our lake deposits. 



Ci> 



Fig. 390 



