52 



Morph 4. Plate xix. 



Trinidad, California. This area is located about ten miles 



north of Areata, in the Humboldt Bay region. 



This morph is suggestive of the more primitive type in the 

 species, as is indicated by the well defined secondary spirals 

 and the very faint expression of the vertical lamellae. A ten- 

 dency to develop the high spire characteristic of more north- 

 ern forms is also apparent. The relationship of this morph to 

 those found in San Francisco Bay, Willapa Bay and elsewhere 

 in the southern range of the species is noteworthy. 

 Specimens available, 25. 



Morph 5. Plate xx. 



Alameda, San Francisco Bay, California. 



Although the writer has available for study only a set of 

 six specimens from this area, considerable range of variation 

 is indicated, including forms with short spires and incipient 

 vertical lamellae very suggestive of the types that dominate 

 much of the Puget Sound area. The report by Packard pub- 

 lished in 1918 illustrates a number of divergent forms which 

 indicate the existence in the bay of several distinct local races, 

 although some of these may have been affected by the exten- 

 sive engineering in this body ot water. 



Morph 6. Plate xxi. 



Port Stanley, Lopez Island, San Juan Islands. 



This population is introduced to illustrate the extensive 

 variation frequently found in local races of lamellosa, which 

 indicate the probable influence of nutritive conditions in the 

 environment determining the size of the snails on attaining 

 maturity, as indicated by the completion of the lip protecting 

 the aperture. It will be noted that in this population all of 

 the individual snails are very uniform as regards form, sculp- 

 ture and coloration, indicating they are not only phenotypes 

 but also genotypically similar. Within an area inhabited by a 

 population there are often niches which may differ in avail- 

 able food supply, and thus retard or accelerate the growth of 

 individual snails. Genetic factors may also be involved. 



