PERNA. 



tions; it is " dents sulciformes for, in fact, if the word 

 " teeth'* is at all applicable in this Genus, it raust be ap- 

 plied to the interstices between the grooves, and not to the 

 grooves themselves. 



These shells are nearly equivalve, fiat, and rather 

 irregular, thickish, externally lamellar, the laminae cow(^ 

 posed of minute perpendicular fibres ; their umbones are 

 small, nearly ecjual and placed at the posterior extremity 

 of the hinge margin. Hinge linear, marginal, with nu- 

 merous (sometimes not more than three or four) transverse, 

 parallel, opposite grooves, which, as well as the flattened 

 ridges between, receive the ligament. A posterior sinus 

 for the passage of the byssus, by which it is attached to 

 submarine rocks, «&c. placed immediately beneath the ex- 

 tremity of the hinge margin, with parietal callosity, and 

 more distinct in the right hand valve than in the other. 

 The inner pearly part of the shells extends itself nearly in 

 the same shape as the outer fibrous portion, but not quite 

 so far. There is one distinct, rather irregular, oblique 

 muscular impression, and a row of small dots which serve 

 as points of attachment for a part of the mantle, placed at 

 the posterior side near the sinus for the passage of the 

 byssus. The anterior extremity of the hinge is narrower 

 than the posterior part, and is frequently produced into the 

 form of a pointed lobe, but this is by no means to be con-- 

 sidered a character of the Genus, any more than the simi- 

 lar, sometimes very acute lobes in Malleus are, because it 

 is not essential, and several species have it not. 



We have mentioned under Crenatula, the characters- 

 by which Pernae may be distinguished from it, therefore, 

 shall not here repeat them. 



The Pernae are marine, the recent species are brought 

 from the Indian Seas and from New Holland. Several 

 fossil species are known which occur in London Clay, Cal- 

 caire grossiere, Clunch Clay, &c. 



Fig, 1. Perna tsognomon. 



2. . Ephippium. 



3. Francii. 



4. to which, being only the cast of tfie inside, we do not give a 



- specific name. 



5. — maxillata. 



