BIVALVIA. 21 



AMUSIUM (sp.). Chem. 1784. 



PALLIOLUM. Id. 



PYXIS. Id. 



PERA. Id. - P. opercularis. 



PALLIUM. Martini, 1773. Schum. 1817. 



DENTIPECTEN. Eiippell, sec. Gray. 



DECADOPECTEN. Ruppell, sec. Swains. 



PSEUD-AMUSIUM. Chem. 1784. P. tigerinus. 



AMUSIUM. Rumph. 1705. Chem. 1784. Schum. 1817.' 



PLEUEONECTES. Schlotth. 1820. 



PLEURONECTIA. Swains. 1840. P. Gerardii. 



Generic Character. Shell ovate, or suborbicular, subequilateral, inequivalve, with 

 a projecting, generally unequal auricle, on each side of the umbo, surface rayed with 

 striae, or more or less elevated costse ; beaks approximate and acute. Hinge with a 

 linear groove across the dorsal part of the auricles for the ligament, and a triangular 

 pit or fossette beneath the umbo for the cartilage. Muscular impression of the mantle 

 entire but ill denned ; adductor large, excentric. 



Animal resembling the shell without the auricular appendages, its mantle dis- 

 connected on all sides ; the margins bearing generally two rows of tentacular filaments, 

 at the base of which are arranged a series of ocelli or rudimentary eyes ; foot small, 

 subcylindrical, containing a groove from which is spun a byssus for its attachment ; 

 one large and powerful adductor muscle : no siphonal tubes. Sexes united. 



This is a well and strongly-marked genus, generally ornamented with rays, like the 

 expanded sticks of a lady's fan : these in some species are large and highly elevated, 

 while in the more aberrant forms they are nearly obliterated, merging into some which 

 are perfectly smooth, the gradations from the one to the other are so imperceptible as 

 to be without any definable line of demarcation, and although this group has been 

 separated into several Genera, there is no good or permanent character by which they 

 can be distinguished. 



In some, the valves are very unequal in size, while in others there is scarcely a 

 perceptible difference, though in most species, by close observation, a trifling 

 inequality may be detected. Those, in which one valve is very convex, while the 

 other is flat, or even concave in its young state, have mostly the larger one buried in 

 the sand, resting in a horizontal position, so that the flat one opens upwards, like the 

 lid of a box, these, when young, are furnished with a byssus, the opening for which 

 is visible in the smaller shell, but becomes obliterated in the adult. The animal 

 probably, in all the species, is capable of producing a byssus for attachment, this, 

 however, is mostly made use of by the young, as when more advanced in growth it 

 appears to be possessed of a considerable degree of locomotive power, which is often 

 employed so as to make considerable progress through the water by means of its large 

 and powerful adductor after opening the valves, and flapping them suddenly together 

 with great rapidity ; some species are probably always stationary, as we find at all 



