Mr. J. Lycctt on Pcrna 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 Pinna and My tilt ; 

 tin- li'mire-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. 



I lu'diuvtl 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 lanrrr valve ; the surface, unlike that of 

 the typical Per/i#, is smooth ; the right valve has little con- 

 \c\ity, and its umbo little prominence; its anterior bord- 

 thickened as in the other valve. 



In the Periuf, as in the fnocera.m, much variability exists in 

 VOL. n. K 



