30 SIXTEENTH REPORT. 



Physa ancillaria Say, var. Two specimens were obtained which appeared 

 to be close to this species. They were quite dark brownish horn in color, 

 with a purplish brown callus. The largest example measured: altitude 11.7 

 mm., diameter 7.3 mm., aperture length 9.3 mm., while a specimen of Physa 

 ancillaria parkeri, from the shore of the lake, measured: 12.3 mm., 9.2 mm., 

 10.5 mm. 



Secondary species. 



Segmeniina crassilahris Walker. Extremely abundant. 

 Musculiurn securis (Prime). Abundant. 

 Planorhis hirsutus Gould. 

 Pisidium variahile Prime. Rare. 

 Lymnaea dbrussa Say. Very rare. 



b. Comparison of Different Pools. 



To bring out more clearly the relationships of these different pools, two 

 charts were prepared: one, (chart 2), comparing the four lagoons and pools 

 of Douglas Lake, while the other (chart 3), the pools of three different regions 

 in northern Michigan. Six pools were shown in this last chart; the first 

 four from Huron County (5), the fifth from Dickinson County (7), and the 

 sixth the Sedge Point Pool of Douglas Lake. 



The pools in the first class were the remnants of small, sand-spit lakes on 

 the south side of Sand Point; the fourth was "Long Lake," a barrier beach 

 pool on the north side. The pools on Stony Island were formed on a large 

 sand spit which, in fact, constituted most of the island, but were separated 

 from Saginaw Bay by what appeared to be a barrier beach; they were on the 

 less protected side of the island. On the other hand, the pools near Rush 

 Lake in Huron County, and the sedge pool of Dickinson County, were 

 formed in connection with streams. The first were apparently creek-flat 

 pools; the second was a glacial depression which was close to the Sturgeon 

 River, and certainly drained into the latter, during high water, if it was not 

 flooded by it. 



From chart 3, it will be seen that the characteristic pool molluscs of these 

 portions of northern Michigan were: Planorhis trivolvis among the larger 

 species, and Musculium securis among the smaller ones. The presence of 

 some medium-sized Lymnaea was also quite typical; in the Sand Point and 

 Stony Island pools, Lymnaea lanceata, derived from the lagoon formations 

 of Saginaw Bay, was quite common, while in the pools near Rush Lake and 

 in Dickinson County, Lymnaea palustris, with varieties, and Lymnaea kirt- 

 landiana, respectively, appeared to be the pioneers of the temporary swamp 

 conditions (see chart 4). Lymnaea exilis seemed to be the example of this 

 type in the Sedge Point Pool, while both in it and in the sedge pool of 

 Dickinson County, Lymnaea stagnalis appressa, a shell more characteristic 

 of larger bodies of water, was very numerous. Another characteristic genus 

 was Segmeniina, one or the other of the two species being usually present. 



It will be noticed that, in this paper and in former papers by the writer, 

 the line between lakes and pools was apparently drawn farther along than 

 by Shelford (13), or by F. C. Baker (1) ; in fact, the pools of the writer appear 

 to correspond to the senescent stages, at least in the molluscan fauna, of 

 those described by Shelford. The writer did not find, in any of the three 

 regions mentioned, Planorhis campanulatus or Amnicola limosa in any pool 

 closed off from the parent lake or stream, and the Sedge Point Pool was the 



