MOLLUSC A. 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, irreguliere, 

 adherente, a crochets court, enfonces dans la base des 

 valves et deriges de cote. Charniere sans dent ; deux im- 

 pressions musculaires separees et laterales. Ligament demi- 

 interieur, enveloppant une callosite oblongee, et sortant en 

 dehors par une fissure recourbee." 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. 



\ 3. ARC A. Linnaeus 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 constitute a very natural family. But in this family 

 there are several groups of which Bruguiere formed sections 

 and Lamark genera. The genus area 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, note, tetra- 

 gona, barbata, andfusca, are natives of the British Seas. 



The Linnaean arcae, which have the hinge line broken 

 and angular, belong to the genus NUCULA. In this genus 

 the beaks are contiguous and turned a little backwards. The 

 nucula, nuclea, minuta, rosfrata, and tenuis, are found on 

 our shores. 



In the genus PECTUNCULUS, the hinge teeth are situated 



