Dr. J.E. Gray ow some Families of Bivalve Shells, 367 



is always to be distinctly seen under the apex of the umbo, even 

 in those species oi Area, like A.Noce, which have a linear series 

 of numerous nearly uniform plates. 



In most species the two teeth or groups of cross-plates are 

 of nearly uniform size and disposition ; but in some the anterior 

 tooth is very small, as in the genus Argina, and in the allied 

 genus Lunar ca the small anterior tooth is entire, and not divided 

 into cross-plates ; in Litharca the hinder tooth is entirely want- 

 ing, the anterior tooth being like that of the true Area. 



The ligament is external, covering the area or talus between 

 the umbones, formed by the gradual thickening of the cardinal 

 edges, which causes the umbones to separate further and further 

 from each other as the shell enlarges. The cartilage is situated 

 in a series of small pits on the outer edge of the hinge-margin, 

 these pits being moved forward as the shell increases in size, 

 leaving grooves diverging from the apex of the umbo towards 

 the margin of the shell ; the grooves on the two valves forming 

 a circumscribing series of concentric lozenge-shaped areas on 

 the talus. In some genera, as Senilia, the grooves are complete 

 and close together ; in others, as the true Area, the grooves of 

 the young shell are complete ; but as the shell increases in size, 

 the grooves very often do not reach to the umbo, but look like 

 a pair of lines regularly diverging from the hinge-margin. 



In the first and second sections the cartilage in the very young 

 shell forms a single triangular spot on the hinge-margin, just in 

 a line with the umbo. As the shell increases in size and the 

 hinge-margin extends, the cartilage divides in the centre, the 

 separate parts gradually diverge from each other as the hinge- 

 line extends, shelly matter being deposited between the parts, 

 and a new piece of cartilage is deposited in the place where the 

 otjier was formerly situated; at length this separates in half, 

 like the former ; and as the shell reaches mature age, there is a 

 succession of angular lines placed concentrically one within the 

 other, formed by the successive cartilage-pits, the youngest and 

 smallest being in the centre. 



In the first section, where the talus is large, by the rapidity 

 of the enlargement of the hinge-margin, the cartilage-pits and 

 grooves are far apart ; and in the second, where the talus or area 

 is smaller and less developed, they are nearer together and more 

 evenly concentric. The grooves formed by the cartilage-pits are 

 to be seen on the surface of the talus in the fossil species. 



In some of the species of the third section the cartilage ap- 

 pears to occupy the whole length of the cardinal margin, and 

 each new portion of cartilage and ligament deposited on the 

 hinge-margin is rather longer than the one formerly deposited, 

 so as to fit itself to the extended length of the hinge-margin. 



