THE NAUTILUS. 117 



In August, 1912, Dr. Walter Faxon brought me three specimens 

 of Radix auricularia (Linn.), which he hail found very near the 

 place where I had collected /*. salsa, and 0. bisuturalis and trifida 

 in 1893. Iti October, 1912, Dr. Faxon and I collecting again at 

 this spot, secured several more specimens, varying in age from very 

 young to fully mature. All we found were on Oliara, and in more 

 or less deep and exposed parts of the river. I examined quantities 

 of Potamoyeton without secunng a specimen, but nearly every clump 

 of Cliara yielded at least one. In the shallow sheltered coves and 

 ditches we found great quantities of Galba palustfis, Planorbis 

 trioulvis, Plunorbis tilbns, Pliysn Iteterostropka and Ancy/us parallelus 

 but these species all avoided the deep water of the river as surely 

 as 7?. auricularia kept away from the shallow places. I have never 

 found Galba pnlustris, P. trivolvis, or R. aitricularia in that part of 

 the river above Watertown which has always been fresh water. 

 The common species there are Psendo succinea columella and Pfan- 

 orb/'s bicarinatus, neither of which have yet been found in the river 

 at Cambridge. 



There are numerous explanations for the sudden appearance of 

 exotic species in unexpected localities. A very popular, and probably 

 in many cases a true one, is that they have escaped from some nearby 

 greenhouse. In support of this theory, I will admit that there are 

 several greenhouses in Cambridge, and that from some of them to 

 the river would be but a short walk, but a comparatively long crawl. 



SPH^RIID^E OLD AND NEW, III. 



I5Y V. SrG 



P. COLUMBIANUM, 1 n. sp. Mussel of medium size, strongly 

 inflated, outlines (along the valve edges) oval to elliptic without any 

 angles, beaks somewhat behind the middle, large, prominent, 

 rounded or slightly flattened on top, or even calyculate : surface 

 more or less uneven from lines of growth, somewhat shining, with 

 fine irregular stria?, color light corneous to yellowish, often in alter- 

 nating zones, shell thin, sub-translucent ; hinge mther slight, cardinal 

 teeth small, the right curved with the posterior end thicker, left 



'The name is not strictly in conformity with the rules, but appears prefer- 

 able to columbiense. 



