THE NAUTILUS. 117 



III August, 1912, Dr. "Walter Faxon brought me three specimens 

 of Radix aariculdi'ii (Linn.), which lie luul found very near the 

 place where I had collecK.'d P. snlsn, and 0. bisutnratis and trifida 

 in 1«93. In Octoher, 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 Cliura., and in more 

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

 of Potninoijefoii without secn>"ing a specimen, but n«^arly vxary clump 

 of Cluira yielded at least one. In the shallow shellered coves and 

 ditches we found great quantities of Gulba pnlustris, Phinnrhis 

 trivolois, Pl'inorbis albus, Pliysn heterostroplui and Ancylus pnralldus 

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

 as R. auricularin k<*pt away from the shallow placi'S. I have never 

 found Gulba pu/ustris, P. frivoloi^, or R. (turicniaria in that part of 

 the river above Watertown which has always been fresh water. 

 The common species there are Psendo succlnca coluinelld and Plmi' 

 orbis bicarinatuSf neither of which have yet been found in the river 

 at Cambridge. 



There are numerous explanations for the sudden appearance of 

 exotic S[)ecies in unex|)ected localities. A very po|)ular, and prohalily 

 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 fiom sonie of them to 

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



SPHJEElID^ffi OLD AND NEW, III. 



HY V. srEUKI. 



P. COLUMliiAXUM,' n. sp. — Mussel of medium size, stronnrly 

 inflated, outlines (along the valve j^lges) oval to elliptic witlioiit any 

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

 rounded or slightly Haltened on lop, or even calyculate: surface 

 more or less uru-ven from lines of growth, somewhat shining, with 

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

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

 teeth small, the lii^ht cin-ved with lh« posterior end thicker, left 



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

 able to columbiense. 



