MOLLUSCA. 87 
from species brought from the Indian Seas. He terms it 
etheria, and originally described its generic character in the 
following words : “ Coquille bivalve, inequivalve, irreguliére, 
adherente, a crochets court, enfonces dans la base des 
valves et deriges de cote. Charniére sans dent ; deux im- 
pressions musculaires separ¢ées et laterales. Ligament demi- 
interieur, enveloppant une callosite oblongée, et sortant en 
dehors par une fissure recourbée.” He has described four 
species which are very rugged on the outside, but finely 
nacred within, and has placed the genus in his family ca- 
macea, While in external aspect, and in the absence of 
teeth, the species make a near approach to the ostreacea. 
13. Arca. Linneus assimilated, under this genus, every 
shell the hinge of which presented numerous mutually inserted 
teeth. The shells which were thus united, have numerous re» 
lations, and constitutea very natural family. But in this family 
there are several groups of which Bruguiére formed sections 
and Lamark genera. The genus arca is now restricted to 
those shells in which the hinge is in a straight line, and 
composed of numerous small lamelliform teeth, without la- 
teral ribs. They have obtained their name from their re- 
semblance to a ship, when the shell is inverted. Many 
species of this genus gape a little at the superior margin, to 
enable the animal to send out those tendinous threads by 
which it adheres to the rocks. The A. lactea, noe, tetra- 
gona, barbata, and fusca, are natives of the British Seas. 
The Linnean arce, which have the hinge line broken 
and angular, belong to the genus Nucuxa. In this genus 
the beaks are contiguous and turned a little backwards. The 
nucula, nuclea, minuta, rostrata, and tenuis, are found on 
our shores. 
In the genus Pecruncu.vs, the hinge teeth are situated 
