514 



LECTURE XXI. 



in successive layers, tiie innermost being the largest and latest 

 formed ; and each layer presenting a cellular lamellar or prismatic 

 texture, which presents characteristic vax'iations in different families. 

 Each valve of the normal shell is a cone, showing every grade of 

 depth, from the flat plate of the Placuna, to the produced and spiral 

 cavity of Isocardia and Diceras ; it is commonly shallow, with the 

 apex or umbo turned more or less to one side and directed ybrit-arc?*. 

 If you place a bivalve shell in the position of the Mactra {fig. 192.), 

 b is the umbo, and de- 

 termines e as the ante- 

 rior border ; g, there- 

 fore, is the posterior 

 one, a the upper or 

 dorsal, and/ the lower 

 or ventral bordei*. The 

 length of the shell is 

 taken from e to g ; its 

 breadth, at right angles, 

 from the dorsal to the 

 ventral border, /; its 

 thickness is measured 

 across the closed valves, 

 at the most prominent Mactra. 



part, from the right to 



the left side of the animal. Transfer yourself, in imagination, within 

 the shell {fig. 192.), with your head towards e, and your back 

 towards a and b, and you will recognise the ralve figured, as the 

 right valve. Anterior to the umbo b, there is usually an oval de- 

 pression, forming a concavity in the outline of the valve ; it is called 

 the liiniila c. The hinge-ligament is sometimes between the umbones, 

 never anterior to them. If the shell be divided by a line dropping 

 from the umbo into an anterior (e f) and posterior {fg) part, 

 it is never equally divided ; in other words, it is inequilateral. 

 Pectunculus is least so : in Glycymeris and Solemya the anterior 

 moiety is longer than the posterior one ; in almost all other bivalves 

 it is shorter, as in fig. 192., and commonly much shorter. Most 

 Lamellibranchs are equivalve, that is, the right and left valves are of 

 the same size and shape. The exceptions occur in the stationary i 

 and often fixed species, which lie on one side, when the lower valve is ' 

 deeper and more capacious than the upper one : this lower valve in the 

 oyster, Pandora, and Lyonsia is the left valve, the smaller and flatter 

 upper valve is the right one : in Chamostrea and Corbula, the left is 

 the smallest valve. Tlie Placunce, Pectenes, Spondyli, and AvicuUdce 



