Mr. J. Lycett on Perna quadrata. 119 
advance of growth; the valves are so much flattened, that their 
attenuated apical extremities have not more thickness or con- 
vexity than the corresponding parts of the Pinne and Mytili ; 
the hinge-plate is always broad, the greater length of the valve 
being always perpendicular to, or in the opposite direction to 
the line of the hinge-plate. Perna quadrata, on the contrary, 
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 
Perna quadrata, Sow. 
1. Exterior of the convex valve. 2. Hinge-plate of the flattened valve. 
(Reduced one-fourth.) 
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 Perne, 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 
VOL. 11. K 
