58 XINETERNTH REPORT. 



being an important factor in determining the abundance of a species in 

 a particular area, granting that local conditions were favorable to that 

 species. It was noted repeatedly that large Scirpus associations in 

 rather shallow water, if adjoining a shore near a lily association were 

 favorite habitats for young frogs of series 1, but small, isolated Scirpus 

 associations, even if in shallow water, contained few frogs. 



The primary species of reptiles of series 1 are three semi-aquatic 

 snakes — the watersnake, the ribbon snake and the common gartersnake. 

 As these three species of snakes bear living yoinig, the breeding habitat 

 is not a limiting factor in their local distribution. In this respect they 

 differ from the primary amphibian species. The food of the water- 

 snake includes fishes, both living and dead, small frogs and tadpoles. 

 Other animals are also taken, but those listed form the most important 

 portion of its food in the Douglas Lake region. Many of the Avater- 

 snakes examined contained fish in the alimentary canal. In captivity 

 watersnakes fed upon young frogs. Twice in the field this species of 

 snake was seen to capture small frogs and swallow them. The water- 

 snake was not as dependent upon the ])rimary amphibian species of 

 series 1 as were the two species of Thamnophis. These snakes, Tham- 

 uophis sauritus and Thamnophis sirtalis were observed many times in the 

 field feeding upon young frogs, and the stomachs of several specimens 

 dissected at the Biological Station were found to contain young frogs. 

 Thamnophis sirtalis at least was not entirely dependent upon the primary 

 amphibian species of series 1 for food, for on three occasions small 

 gartersnakes of this species, less than two feet in length, were found 

 choked to death by small perch, Perca fiavescens, which had been partly 

 swallowed, suggesting tliat the common gartersnake makes some use 

 of small beached fishes for food. From the tables it may be seen that 

 the two species of Thamnopliis in general frequented the same habitats, 

 in which they were about equally abundant, although considering all 

 habitats of the region, the common gartersnake was the more abundant 

 of the two. Usually the two species were not as definite competitors of 

 one another as their habitat distribution might imply. Thamnophis sauri- 

 fii.s, tlie ribbon snake, frequented the grassy portions of the liabitats in 

 wliieh it was found and. Thamnophis sirtalis, the common gartersnake, 

 the more open, moister parts. It was also found in the water more fre- 

 quently than the ribbon snake. In tlie old. almost filled beach pools 

 and in the more comi)letely covered portions of the Cliamaedaphne bogs 

 Tliamnophis sauritus was the more abundant species. This intra-asso- 

 ciational distribution of these two snakes seemed definite, although 

 detailed data are not at liand for a complete comparison. 



