242 PALAEONTOLOGY. 



The area, or posterior slope, fig. 7, r, is that part of the 

 shell in which the ligament is placed. 



The areola, or anterior slope, fig. 7, s, is the space oppo- 

 site to that in which the ligament is placed. 



The umbones, fig. 7 and 8, are the prominent points of the 

 dorsal margin. They vary much in form, being proximate or 

 remote, straight or bent, in the different families ; in Pedun- 

 culus, they are straight ; in Venus, curved towards the 

 anterior margin ; in Isocardia, spiral; in Chama, decumbent; 

 in Diceras, free. 



The hinge, fig. 8, u, is the point of the dorsal margin 

 at which bivalve shells are united, either by the teeth of 

 one valve inserting themselves between those of the opposite 

 valve, or by a process of one fitting into a cavity of the other. 

 The construction of the hinge, in conjunction with the 

 general contour of the shell, affords generic characters for this 

 class. 



The ligament, fig. 2, v, is that flexible, cartilaginous sub- 

 stance by which the valves are united, and the shell opened, 

 by means of its elasticity. It is always situated under the 

 umbones. 



The disk, fig. 7, w, is the convex centre of a valve, or its 

 most prominent part. 



The base, or ventral margin, fig. 7, y, is the part imme- 

 diately opposite the umbones. 



The height, fig. 9, z, is measured from the umbo to the 

 ventral margin. 



The length, fig. 9, a', is calculated from the extreme edge 

 of the anterior and posterior slopes, being in a contrary 

 direction to the height. 



The auricles, fig. 9, b r , b', are the processes on each side of 

 the umbones. Some species of pectens have one ear very large, 

 and the other extremely small ; while some are scarcely dis- 

 cernible on one side. 



The valves of a multivalve shell are depicted in fig. 10, e'. 



The lunule, fig. 2,^, is the small depression on the dorsal 

 margin, anterior to the umbones. 



The corselet, or escutcheon, fig. 2, g', is the larger de- 

 pression posterior to the umbones. 



There are a few other terms which may require explana- 

 tion ; thus the muscular impression is the mark made by the 



