92 



Genus 1. ARCA, Linnams. 



Animal ; oblong ; mantle freely open, simple or fringed, no siphons ; 

 foot large, oblong, bent, grooved throughout its length so as to 

 form a disk, with plain or slightly crimped margins, a byssal 

 gland at its base, byssus compact ; mouth surrounded by labia 

 formed out of the extremities of the branchiae, no true palps. 

 fForbes.J 

 Shell ; sometimes equivalve, sometimes inequivalve, ventricose, 

 mostly radiately ribbed, covered with a horny or fibrous hairy 

 epidermis; umbocs prominent, more or less distant from each 

 other, according to the width of the ligament area ; hinge linear, 

 straight or slightly arched, with the teeth small and very nu- 

 merous ; ligament external, attached superficially to the area 

 between the umboes. 



The Arks or boat-shells comprehend two sections of very distinct com- 

 position and sculpture, arising out of a difference in the habit of the ani- 

 mal. The Area proper have a strongly ribbed shell of marble-white sub- 

 stance, enveloped by a horny or fibrous epidermis, and its mollusk is said 

 to acquire motion by the contraction and expansion of its foot, after the 

 manner of a snail. They live consequently free, and in some instances 

 have shells remarkably solid and ponderous. Another feature of this group 

 is that the right valve of the shell is frequently smaller than the left, drop- 

 ping a little way into it, and it is always less elaborately sculptured, as if 

 the right mantle-lobe of the animal were weaker in its functions than the 

 left. The section of Bgssoarcce, as the name denotes, live attached to rocks 

 by a byssus, which passes from the animal's foot through a rude gaping of 

 the shell; and the shells of this group are of much lighter substance, and 

 the hinge is more simple, the teeth are smaller, and the ligament is pro- 

 portionally weaker. In all the species of this genus the hinge is composed 

 of a somewhat irregular set of plaits, but the valves very closely interlock 

 with them. 



The Arks are of world-wide distribution, but the greater number are 

 inhabitants of the western shores of South and Central America. The 

 British fauna includes three species. 



Species. 



1. alternata, Reeve. 3. Americana, Gray. 5. angulata, King. 



2. ambig.ua, id. 4. angicostata, Reeve. 6. anomala, Reeve. 



