98 AVICULIDiE. 



(contained in the ' Zoological Miscellany' for 1814) lie 

 mentioned that " one indigenous species " had been dis- 

 covered at Plymouth by Mr. Prideaux. In his ' Synopsis 

 of the British Mollusca' he gave the last-mentioned 

 species the name of Britannica. 



Pema alata (Crenatula Travisii of Turton) is a native 

 of tropical seas, but was accidentally imported into this 

 country on the bottom of a ship which came into Scar- 

 borough. Many foreign mollusca have been introduced 

 in the same way. They continue to live for some time 

 after entering our colder seas, but they never become 

 acclimatized. 



Genus II. PINNA *, Lister. 

 Frontispiece and PL III. f. 1. 



Body oblong and attenuated : palps small : byssus silky and 

 copious. 



Shell forming an elongated triangle, equivalve, widely 

 gaping in front, and slightly on the anterior side for the passage 

 of the byssus : margins entire : beaks terminal and pointed : 

 hinge toothless. 



We now approach the Mussels, to which the shells 

 composing the present genus bear a considerable resem- 

 blance. The principal distinction is that the former 

 have the valves entirely closed, while in the latter they 

 gape widely at the larger end, as well as that the beaks 

 in Mytilus and its allies are not placed at one end of the 

 shell as in Pinna. According to Da Costa the shell of 

 Pinna is called in France "jambon" and "jambon- 

 neau" ; and it looks exceedingly like a small ham. In 

 another point of view it is a wingless Avicula. Pliny's 

 account of the little pea-crab, which is so often found in 

 this mollusk (as well as in Mytilus modiolus and Cyprina 



* From the irivva of Aristotle, who first mentioned tins mollusk. 



