144 CLASS I. TR0P10P0DA. ORDER II. UNIMUSCULOSA. 



The shell of the Aviculacea may be described as being irregular, thin, 

 fibro-laminal, and sometimes foliated on the outside. The hinge is eden- 

 tulate, having a strong ligament inserted in one or more variously-shaped 

 pits, and the muscular impression is sometimes compound. The animal 

 fixes itself by a byssus, which passes out through a notch in the hinge or 

 front margin. They are divided into five genera, as follows : 



Crenatula. Vulsella. 



Perna. Avicula. 



Malleus. 



CRENATULA, Lamarck. 



Testa tenuis, suba?quivalvis, complanata, fibro-laminaris, subirregularis. 

 Cardo lateralis, linearis, marginalis, crenulatus ; crenis in seriem 

 ordinatis, callosis subexcavatis, ligamentum excipientibus. Im- 

 pressio muscularis oblonga, indistincta. 



Lamarck and De Fe'russac both agree in placing the Crenatulae at the 

 commencement of this family, because of their affinity with the Pinnce, 

 the present genus being established by the first of these authors on 

 account of a peculiarity in the hinge. In the shell of Pinna the hinge 

 margin is simple ; whilst in that of Crenatula it is so crenulated as to 

 form a series of distinct concavities ; each cardinal concavity contains 

 a separate portion of the ligament, and thus forms a transition to the 

 deep parallel grooves, so characteristic in the shell of Perna. There are 

 several species of this interesting genus, one only of which appears to 

 have been known to the early naturalists ; by some it was arranged with 

 the Ostrea, by others with the Pinna. 



The shell of Crenatula may be described as being thin, nearly equi- 

 valve, smooth, fibro-laminar, and rather irregular. The hinge is lateral, 



