G4 PALEONTOLOGY. 



P ; its height or breadth at right angles from the dorsal to 

 the ventral border ; its thickness is measured across the closed 

 valves, at the most prominent part from the right to the left 

 side. Transfer yourself in imagination within the shell (fig. 

 15), with your head towards A and your back towards the 

 dorsal border, and you will recognise the valve figured as the 

 right valve. Anterior to the umbo there is usually an oval 

 depression, forming a concavity in the outline of the valve ; 

 it is called the lunula. The hinge ligament is sometimes 

 between the umbonas, never anterior to them. If the shell be 

 divided by a line dropping from the apex of the umbo into an 

 "anterior" and "posterior " part, it is never equally divided ; 

 in other words, it is unequilateral. Pactunaulus is least so ; 

 in Glyaymaris and Solamya the anterior moiety is longer than 

 the posterior one ; in almost all other Bivalves it is shorter, 

 as in Cytharaa ; commonly it is much shorter. Most Lamelli- 

 branchs are equivalve ; that is, the right and left valves are of 

 the same size and shape, as in Cytharea (fig. 15). The exceptions 

 occur in the stationary 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 oysters (Ostrea), in 

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

 upper valve is the right one. In Ohamostrea and Corbula the 

 left is the smallest valve. The Plaaunce, Paatinas, Spon- 

 dyli, and Amanlidm rest on the right valve ; the Anomioe are 

 attached by degenerated muscular fibres passing through a 

 hole or notch in that valve to a more or less calcified lamellar 

 plug. All these shells are called inequivalve. 



The bivalve is called alose when the valves fit accurately ; 

 it is gaping if part of the borders do not come into contact 

 when the shell is shut. In Gastroalimna this permanent 

 opening is anterior, and serves for the passage of the foot. 

 In Mya it is posterior, and serves for the passage of the 

 byssus ; in Sokn and Glycymcris the shell gapes at both ends. 



