66 TESTACEA. 



MTTILUS POLYMORPHUS (Dreissena polymorpha). Plate XIII.*, Figs. 1, 2. 



Whether this animal should be included here is questionable, for 

 naturalists affirm it to be of foreign origin, and accidentally brought to 

 our harbours, docks, and canals, thence not a native species. Likewise 

 its introduction is alleged to have been but of recent date. Whether 

 these be facts, the animal is now naturalized in the canals of Scotland, as 

 in those of England. Nor can much doubt be entertained that other 

 Testacea are in the same predicament. 



This species extends ten lines by six ; and is less than six lines thick. 

 Its shape is somewhat like a stout wedge reversed, the acute angle above. 

 Colour grey, with short yellowish streaks. The margin of the animal is 

 fringed ; and, like the preceding mussels, where broader, may be arranged 

 as if forming two cylinders, rising a line, dark and speckled. A whitish 

 foot protrudes half an inch. 



This is a very inactive animal. Dwells in fresh waters. At least the 

 specimen represented is from a canal. 



PLATE XIII. 



FIGS. 1, 2. Mytilus polymorphus (Dreissena polymorpha). 



2. MODIOLA. 



Modern naturalists have dismembered the Mytilacean tribe of olden 

 authors, to constitute a distinct genus under the name of Modiola. 



MYTILUS MODIOLUS Linn. (Modiola modiolus) The Horse Mussel. 



Plate XII., Figs. 3, 4. 



This is the largest of the race of mussels inhabiting the Scotish seas. 

 The shell extends five inches and three quarters, by three inches ; and, 

 together with its contents, forms a heavy compact subject. Colour 

 brownish blue. The animal is large and fleshy. No marginal fringe is 



