146 CLASS I. TROPIOPODA. ORDER II. UNIMUSCULOSA. 



and this arrangement has been universally adopted, not only on account 

 of the existence of a byssus, but because of the marked peculiarity of 

 structure which the shell exhibits in the hinge. The hinge consists of a 

 broad flat surface in each valve, 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 ligament. The ridges which are left by 

 the cutting of these grooves do not interlock, but shut flat upon each 

 other, the ligament occupying the interstices. These, therefore, may be 

 called ridges, not " sulciformed teeth," as described by Lamarck, as they 

 are in no way analogous to the row of teeth which characterize the shells 

 of the Arcacea (vide p. 104). The hinge of that family, however, differs 

 so widely, that we should not have thought it necessary to refer to it, had 

 not our attention been arrested by Lamarck's erroneous application of 

 the term "teeth" to the cardinal ridges in the shell of Perna. The Area 

 and the Pernse are totally distinct, whether as regards the growth and 

 composition of the shell, or the habits and general anatomy of the ani- 

 mal ; they are not indeed referable to the same natural order. The fibro- 

 plated structure of the shell of the Aviculacea is very characteristic in the 

 present genus ; the animal occupies but a small central portion of the 

 valves, and the byssus passes through a notch on the posterior side of 

 the hinge. 



The shell of Perna may be described as being flat, fibro-laminar, nearly 

 equivalve, and rather irregular, the umbones being small and nearly 

 equal. The hinge, which is broad and marginal, is divided into a num- 

 ber of parallel grooves, destined for the reception of the ligament ; the 

 valves are thus powerfully united, and give the animal but a limited 

 sphere for the opening of its shell. There is always a marked posterior 

 sinus near the hinge for the passage of the byssus, and it is lined with 

 hard layers of testaceous matter, which increase with the growth of the 

 shell. The muscular impression is of an oblique form, and generally 

 distinct. 



The Pernse are very prolific, and are often found in considerable clus- 

 ters attached firmly to each other by their byssi. 



