270 INTRODUCTION TO CONCHOLOGY. 



muscular impression is sometimes compound. The mollusk 

 fixes himself with a bjssus, which passes out tlirough a 

 notch in the hinge or front margin. 



The Aviculacea are divided into five genera — Crenatula, 

 Perna, Malleus, Vulsella, and Avicula. 



Several species belong to the genus Crenatula, but only 

 one appears to have excited the attention of early naturalists. 



The genus Perna possesses a byssus, and is remarkable 

 for a peculiarity of structure which the shell exhibits in the 

 hinge. The hinge consists of a broad flat surface in each 

 valvCj cut across by a parallel series of grooves^ in each of 

 which^ as in the hollow concavities in the shell of Crenatula, 

 is inserted a separate portion of the Hgament. But the 

 ridges wliich are left by the cutting of the grooves do not 

 interlock, but shut flat upon each other, the ligament occu- 

 pying the interstices. Perna is very prolific, and is found 

 in considerable clusters. 



Concerning the Vulsella no particulars of interest are 

 recorded ; the shell is of the same thin, fibro-laminal compo- 

 sition as those of its bretliren, but the animal remains un- 

 knovTU. Neither has it yet been ascertained whether a 

 byssus pertains to the genus ; most probably not, because 

 Vulsella are generally found in sponges. 



