260 AMPHIBIANS AND REPTILES. 



with the aquatic habitats are air breathers during a part or all of 

 their existence, and few are peculiar to those habitats. S'ome of 

 them are terrestrial except in the larval state or during the breeding 

 season (Hylidae, Ranidae, Ambystomadae), while others are always 

 air breathers but live in the water except during the breeding season 

 (Testudinata), and one other Natrix sipedon, only takes to the water 

 to obtain food and escape enemies. Diemyctilus viridescens is unique 

 in that it is an aquatic form in the larval state, a terrestrial 

 air-breather the second year, and a partially aquatic breather in the 

 adult state. 



This fauna, not being strictly an aquatic one, is not dependent 

 upon continuous waterways in its migrations. The presence of 

 bodies of water is, however, an indispensible condition in the environ- 

 ment, and the forms, being weak swimmers, prefer the quiet water 

 conditions of ponds, small lakes and bays to those in the large bay. 

 There is a considerable number of ponds, small lakes and quiet bays 

 in the sand region which accounts for the abundance of this fauna. 

 Many of these are transient but they are mostly large enough to 

 presist until after the larval stages of the amphibians that breed in 

 them are passed, and other forms are rarely found in such places. 



B. Swamp habitats. Here may be grouped (a) the grassy marshes 

 that exist throughout the sand region and on Stony Island, and the 

 grass zones about many of the bodies of water, and (b) the timbered 

 swamps. These two kinds of swamps have in common the adult 

 stages of Hyla pickeringi, Bufo americanus (see p. 262), Rana pipiens, 

 and the garter-snakes, T. sirtalis and T. sauritus. Characteristic of 

 the grass swamps are the adult stages of Chorophilus nigritus tri- 

 seriatus, Sistrurus catenatus and Liopeltis vernalis, while Plethodon 

 cinereus, Hyla versicolor, Rana sylvatica cantahrigensis and Ain- 

 by stoma jeffersonianum were characteristic of the wooded swamps. 

 The abundance of these habitats in this region together with the 

 numerous ponds explains the abundance of the forms enumerated 

 above. It should be noted that some of these species are occasional- 

 ly found also upon the sand ridges, these being usually the ones most 

 capable of resisting dessication, e. g., Rana pipiens, Bufo americanus, 

 Liopeltis vernalis, Hyla versicolor, and the garter-snakes. 



C. Sand ridge habitat. As stated elsewhere, the numerous sand ridges 

 are the prevailing feature in the topography of the san d region. These 

 ridges although clothed with vegetation are rather open, and the soil 

 although covered with herbaceous plants is loose and dry. The 

 habitat is thus a dry hot one during the summer months. Amp- 

 hibians are quite scarce here, but the conditions are more favorable 

 for reptiles, and the fact that the number of characteristic forms is 



