THE LAMELLIBRANCHIA 47 



the larger. This may be the left valve, as in the oyster, 

 or the right (ineqiiivalve shells). 



i. Pectunculus. As a first example of a lamellibranch 

 we will take this genus, one very common at the present day 

 and in the Cainozoic era. Familiar species are P. glycimeris 

 of the Red Crag, and P. deletus of the Barton beds. The 

 shell is circular in outline and is lenticular in shape, the 

 valves are symmetrical to one another and very nearly 

 symmetrical in themselves, the distinction between 

 anterior and posterior ends being very slight. In fact 

 the only external distinction in most species is that the 

 umbo is very slightly nearer to the anterior end and faces 

 towards the posterior end, and these facts would not help 

 us much in deciding which end was which, for although 

 in most lamellibranchs the umbo is nearer the anterior 

 end, yet there are exceptions ; and on the other hand 

 the umbo usually points towards the anterior end. 



The external surface is marked by radial and concentric 

 lines : the latter are always very slightly marked, the 

 former vary in different species from very faint striae to 

 coarse ribs. 



Beneath the umbo and above the hinge-line there is 

 an obtusely triangular cardinal area, which in a fossil 

 shell is seen to be bounded above by a slightly raised 

 margin, and to bear a number of ridges arranged like 

 a set of inverted ' Vs,' one within the other. In a living 

 animal this area is concealed by a brown leathery mass 

 uniting the two valves, known as the elastic ligament. 

 (Occasionally this is preserved in fossils.) This elastic 

 ligament is something unknown in Brachiopoda, and 



