132 THE NAUTILUS. 



Virginica somewhat lead color. Interior chalky or greenish white, 

 with a dark violet muscular impression in the centre. 



A. Virginica sometimes attains a length of twelve to fifteen inches, 

 but seldom over three inches in breadth, while in borealis the 

 breadth is about one-half the length ; specimens six inches in length 

 are about three in breadth. 



Oysters have many enemies, among which are sponges, star-fish, 

 drills and man. The drills or borers, Urosalpinx cinerea, are ex- 

 tremely abundant in our bay, and can destroy a great many oysters 

 in a short time. 



Ostrea edulis of Europe is considered by some authors to be iden- 

 tical with our northern oysters. I cannot see enough resemblance 

 in either shell or animal to agree with them. Experienced oyster- 

 eaters can tell the difference between borealis and Virginica in- 

 stantly by the taste, having no knowledge whatever of the shells. 

 Having tried to eat English, Scotch, French and Dutch oysters, 

 judging from that standpoint I could never believe they were the 

 same species as the Ostrea borealis, the finest eating oyster in the 

 world. 



[The following species were omitted in their proper place:] 



175. Pisidium variabile Prime. 



Shell heavy, oblique inequilateral, inflated, anterior longer, nar- 

 rower and angulated at the end; beaks full, prominent, not approxi- 

 mating at the apex ; valves solid, interior light blue ; epidermis 

 glossy, color variable, straw or greenish-brown, with a yellow zone 

 on the basal margin ; cardinal teeth united ; lateral teeth short and 

 strong. Length y 2 ^, height y 1 ^, breadth y 1 ^ inch. 



Described by Temple Prime in Proc. Bost. Soc. Nat. Hist., IV, 

 163, 1851. Inhabits the Eastern and Middle States. A very com- 

 mon species found in nearly all small streams. 



176. Pisidium Virginicum Gmelin. 



Shell thick, oblique, vt-ry inequilateral, coarse and robust in ap- 

 pearance ; anterior rounded, posterior broader, subtruncated at the 

 extremity, beaks large; valves solid, interior light-blue; epidermis 

 greenish-brown to chestnut, with zones of a darker shade ; hinge 

 margin curved; cardinal teeth two, shaped like the letter V re- 

 versed ; lateral teeth short and strong. Length y\\, height y 2 -^, 

 bread tli T 2 ^ inch. 



This is one of the largest species of Pisidium in America. It inhab- 

 its running streams in New England and Canada, and throughout the 

 Middle States, and in a few of the Western States. Say, in 1819, 

 described this shell under the name of Cyclas dubia, and it is gener- 

 ally known to collectors under this name, but Mr. Prime has shown 

 it to be identical -with one described by Gmelin, in 1788, as TeUina 

 Virginica; he, therefore, changes the name to its present one as 

 adopted above. 



