268 



LOWER INVEHTEBllATES. 



of its many vagaries of shape!?, a fact •which has led to its description under a hirge 

 number of generic and specific names. The shell is much like that of Mi/(ilus, but in 



the animal some important differ- 

 ences may be observed. The 

 mantle is closed, leaving only a 

 small ojiening for the byssus, 

 while at the posterior end a short 

 siphon is formed. The most in- 

 teresting fact in connection with 

 Dreissena is its distribution. It 

 is a native of the Aral and Cas- 

 pian Seas, and was discovered by 

 Pallas in 1769 at the mouth of 

 the Volga. Later it was found 

 in the rivers flowing into the Bhick Sea, and from these it is supposed to have been 

 transjiorted into tlie ri\'crs of Germany by the pontoon trains during the Napoleonic 

 wars. Its introduction into England in 1824 is supposed to have been effected by 

 means of foreign timber, and now it is a recognized member of the fauna of almost 

 all parts of Europe. In Ldudcm it has |ii-oved itself something of a nuisance, as it has 

 obtained entrance to the jiijies which supjily the city with water. 



The family Akcad.e embraces thick eqnivalve shells covered with a thick, often 

 hairy, epidermis. The hinge ligament is external, and the hinge itself is very charac- 

 teristic, being formed of a large number of teeth arranged in a single row like those 

 of a comb. Both adductor muscles are present, and equally large, producing corre- 

 si"in(ling imjn-essions on the shell. The edges of the mantle are free, and the gills 

 terminate in free filaments. The foot is large, but very variously shaped. 



Of the genns Area numerous subgenera have been made, and many species com- 

 monly parade under the names Byssoarca, Scapharcu, Arffina, ParalMipedum, etc. 

 For our purpose this refinement of classification is 

 not necessai-y, and we here accejit the genus with 

 the Lamarekian signification. With this classifica- 

 tion we may define Area as a gemis in which the 

 hinge teeth are nearly equal, and form a .straight 

 line, the shell is ventricose, the nmbones arc widely 

 sej^arated, and the free edges of the shell are fre- 

 quently gaping. Area tioce, Noah's ark, received its 



name from some queer fancy existing in the mind of lin- 

 nanis. It secretes itself under stones at low water in the 

 Mediterranean, and closes the gape of its shell with a byssal 

 structure shaped like a cone, and composed of numerous 

 thin plates. Occasionally violet-colored pearls are found in 

 this species. On our eastern coast three s[)ecies occur ; a 

 Acry thick .and lieavy form known as A7xa ponderosa, a 

 more closely ribbed species, A. pexafa, and the longer and 

 Further south other species occur. Area pexata is known 

 as the 'bloody clam,' on account of the red gills and the red fluid with which 

 the tissues are filled. It is covered with a thick, hairy epidermis, and has from 

 thirty-two to thirty-six radiating ribs on the outer surface of the shell. In Area 



^,: - 



tv . 



Fii;. 2SL.'. — An-'f /raust-ersa. 



smaller, A. transversa. 



