428 



J\Ir. J. Lycett on Pema quadrata. 



is very inequivalve, and with advance of growth it becomes 

 almost gryphoidal ; the umbo of the larger valve is very promi- 

 nent, straight and incurved ; the anterior side of the valve is 

 steep, with a large excavation, byssal aperture and corrugated 

 border ; the posterior side is much compressed, and extended into 

 a kind of imperfect wing ; the hinge-plate is narrow, its border 

 is much lengthened, so that the greater length of the valve is in 

 that direction, and the shell is transverse ; the narrow hinge- 

 plate renders the ligamental grooves very short, their diameter 

 laterally being equal to their length, as is often seen in the 

 genus Gervillia ; they diminish rapidly, so that the posterior half 

 of the hinge-line is destitute of hinge-plate and grooves. The 

 byssal aperture is formed by the larger valve only. In both 

 valves the test is very thin, excepting at the prominent umbo 

 and anterior side of the larger valve ; the surface, unlike that of 

 the typical Pernee, is smooth ; the right valve has little con- 

 vexity, and its umbo little prominence ; its anterior border is 

 thickened as in the other valve. 



In the Perna, as in the Inocerami, much variability exists in 



Pema quadrata, Sow. 



1. Exterior of the convex valve. 



2. Hinge-plate of the flattened valve. 



(Reduced one-fourth.) 



specimens of the same species, the result not only of different 

 stages of growth, but also of individual peculiarities. All the 

 specimens of Perna quadrata differ more or less from each other 



