6 GEORGE V. SESSIONAL PAPER No. 39b A. 1915 



III. 

 THE MOLLUSCA OF GEORGIAN BAY. 

 By A. D. Robertson, B.A., University of Toronto. 

 (Plates X— XII) 



In 1910 the writer undertook, in connection with the work of the Biological 

 Station, an analysis of the local molluscan fauna of Go Home Bay. This study 

 has since been extended to include various points around Georgian Bay, but 

 because of the labour involved in working over the material, this paper deals 

 only with the more limited area, leaving the general distribution for future dis- 

 cussion. The analysis of the species is believed to be fairly complete, and special 

 care has been taken to observe critically the specific characters and the variations, 

 whether due to environment, age or other cause. Notice has also been taken of 

 the food of the various forms and of the extent to which they themselves serve 

 as food for fishes and other animals. 



The collections along-shore were made by hand and hand-dredges, while in 

 the deeper water use was made of an iron dredge, provided with a fine inner screen 

 supported by a course outer screen. The latter method entails much labour in 

 sorting over the material but gives excellent results. 



In the identification of species the writer is indebted to Mr. Bryant Walker 

 of Detroit and to Dr. H. A. Pilsbry and Mr. E. G. Vanatta of the Philadelphia 

 Academy of Natural Science, who determined a number of species and confirmed 

 the determination of others. He is also under obligation to Professor B. A. Ben- 

 sley. Dr. E. M. Walker and Dr. A. G. Huntsman of the University of Toronto for 

 much kind assistance and advice. 



The environmental features of this region are of interest because it falls within 

 the Archean area. A general account of these features is given elsewhere by 

 Bensley ('14), but reference may be made to those which are more important 

 from the standpoint of this paper. 



1. The glaciated surface of the rock is sparingly and unevenly occupied by 

 soil and bottom deposits, accumulating in basins and consisting chiefly of dis- 

 integrated particles of gneiss, often with high organic content. 



2. The indented shore-line leaves projecting headlands of fully exposed and 

 bare character, while the protected smaller bays form isolated swamps with usually 

 a deep deposit of mud and much organic material. 



3 The main shore gives place outwards to larger, then to smaller islands 

 and finally to reefs and submerged shoals, with clear rock or boulder bottom. 

 ^ 4. The deeper waters of the larger bays and open waters have flat bottoms, 

 consisting of mud of fairly high organic content alternating with exposed patches 

 of the underlying rock. 



