22 SIXTEENTH REPORT. 



There follows lists with brief notes of the molluscs in the various groups. 

 In these lists the species are divided into three classes according to size: 

 clams, primary species, and secondary species. The notes on abundance 

 refer to the comparative abundance in the groups; this is done in an attempt 

 to more or less equalize the effect of the size in an attempt to prevent the 

 direct comparison of the abundance of a large form, Uke a clam, with that 

 of a small form such as, for instance, Planorhis parvus Say. A small, form, 

 altho very abundant, may not be such a powerful factor in the association 

 as a species less numerous in individuals but of much larger size; it must be, 

 understood, however, that these groups are not meant to express compara- 

 tive dominance, in the sense that this is used in plant ecology. 



The remarks on abundance are purely comparative; figures, however, 

 were obtained for many of the forms by means of a quadrat study, and 

 these will be published in a future report. The following designations are 

 used to express this abundance: very abundant, abundant, quite abundant, 

 very common, common, quite common, frequent, quite infrequent, infre- 

 quent, very infrequent, quite rare, rare, very rare. 



1. Shells of the unprotected, sandy shores. In most places around the 

 south-west end of Douglas Lake, the bottom sloped out gradually to a 

 depth of about a meter, and dropped off more or less suddenly to deep water ; 

 along the unprotected, sandy shores, this "step-off" was particularly steep, 

 and sometimes even attained an angle of forty-five degrees. In these places 

 exposed to the more or less direct action of the waves and currents, the 

 sand was constantly shifting under the action of these agents, and all of 

 the shells were quite large burrowing forms; higher plant life was entirely 

 absent. 



Clams. (All of the clams of the lake were small, approaching the dwarfed. 

 Great Lakes forms.) 



Lampsilis luteola (Lam.). Abundant; from about 20 cm. of water out, 

 mainly in about 1 m. of water near the edge of the shelf. 



Anodonta grandis footiana (Lea). Quite abundant; distribution similar to 

 the preceding species. 



Lampsilis nasuta (Say). Quite common; distribution similar to the pre- 

 ceding. 



Primary species. 



Campeloma decisum (Say). Abundant close to shore in about 15 to 20 

 cm. of water, quite abundant at a depth of a meter, and considerably less 

 numerous between these depths. 



Sphaerium acuminatum (Prime) . Infrequent ; mostly at a depth of a meter 

 or a little less. 



2. Shells of the more or less protected, sandy to marly shores. On the north- 

 eastern shores of the bays and behind spits, etc., the shores were more pro- 

 tected, especially in the shallower waters. The bottom usually contained 

 more or less marl, which often cemented the sand so as to form a rather 

 firm, superficial layer in shallow water, while it was apt to be considerably 

 softer in deeper water. Scirpus americanus usually grew along these shores, 

 with Scirpus validus near the edge of the shelf. There was often a zone of 

 Potamogeton, etc., along the edge of the shelf, while bunches of Chara, etc., 

 were not infrequent in the shallow water of the shelf itself, and Utricidaria 

 cornuta often formed a sort of a sod in the places where the marl helped to 

 form a firm superficial layer. 



