MICHIGAN ACADEMY OF SCIENCE. 31 



only one where clams were found. This partly might have been due to the 

 differences in location; the three regions, described by the present writer, 

 were all considerably north of Chicago, and the colder winters, on account 

 of the greater amount of ice and the resultant stagnancy, might have pre- 

 vented the life of such forms in the shallow waters of the pool formation. 

 It also might have depended partly on the point of view of the writer; I 

 would have called the senescent pools of Shelford the typical pools, perhaps 

 because, in the more northern regions, the transition stages between the 

 lake and pool, which were most prominent in the latter writer's descrip- 

 tions, appeared to be passed thru more quickly. Also, the molluscs present 

 in the "senescent" stages were the only ones common to all pools, no mat- 

 ter what their method of formation. The pools of the writer appeared to 

 resemble more closely the prairie pools of Shelford, perhaps because these 

 apparently did not show transition stages connecting them with the larger 

 lakes, and so lacked relict species from those formations. 



To return to Douglas Lake, the Pine Point Lagoon had about the same 

 shells as the adjoining, marly bottomed shelf (groups 2 and 4), only, as the 

 bottom was more mucky, a few of the mucky bottomed shelf forms were 

 more numerous (Groups 3 and 4). This was what might have been ex- 

 pected from the youngest lagoon of the series; a year or two was not enough 

 time to develop a divergent fauna or to cause a disappearance of the old 

 one. 



The second of the series, in completeness of separation and in age, was the 

 Hook Point Lagoon. Here was found a considerable reduction in the lake 

 forms; the single clam, four of the primary species, and three of the smaller 

 forms had disappeared, while of the remaining ones were less numerous, 

 with the single exception of Amnicola lirnosa, which increased greatly in 

 numbers. This exception may have been due to the fact that the present 

 lagoon was more closely connected with a mucky-bottomed shelf and with 

 the weed zones, while the preceding one was adjacent to a marly-bottomed 

 portion. Two new forms, not including the pisidia on account of the loss 

 of the records of the shelf forms, had appeared in the Hook Point Lagoon, 

 one being common, the other infrequent. The latter of these, Musculium 

 securis has already been mentioned as one of the most characteristic pond 

 forms. 



In the Swamp Point Lagoon was found a still greater reduction of the 

 lake forms. All of the primary species had disappeared as such, with the 

 exception of Campeloma decisum, while two others were sparsely present as 

 juveniles. Planorbis hirsutus was not found, but juvenile specimens of a 

 Planorbis, too young to be identified, appeared partly to take its place. On 

 the other hand, Segmentina crassilahris, another pond species, had appeared. 



The Sedge Point Lagoon differed somewhat from the others; judging from 

 its greater size and probably far greater age, it must have been separated 

 from the main lake much more slowly. All of the lake forms had disap- 

 peared, with the exception of the two clams, which were rare and had prob- 

 ably persisted on account of the large size of the pond.' However, two of 

 the lake shells, Physa ancillaria parkeri and Lymnaea stagnalis perampla 

 (group 4), were represented by changed varieties, Physa ancillaria, var., 

 and Lymnaea stagnalis appressa, respectively. In addition, two new, pri- 

 mary species had appeared, both of them more or less characteristic of 

 ponds; in fact, the first, Planorbis trivolvis, already has been cited as the 



'It is well within the limits of possibility that these were introduced artificially, by students or by 

 fishermen. 



