50 PALEONTOLOGY 



represented by internal and external casts, the material 

 of the internal cast in the two latter cases being 

 rock-matrix, in the first case usually calcite or calcium 

 phosphate, etc. 



On examining the interior of a valve of Pectuncnlus, we 

 see below the cardinal area a curved hinge-plate bearing 

 a row of teeth, all practically alike, usually slightly 

 V-shaped and about twenty or more in number. Com- 

 parison of young and old shells of the same species 

 shows that as growth proceeds new teeth are added at 

 each end of the row, while the cardinal area as it in- 

 creases in size encroaches on the central teeth until they 

 become obliterated. 



Around the margin of the ventral half of the valve 

 is a series of radial grooves and ridges, by which one 

 valve locks into the other when tightly closed. These 

 look very much like a continuation of the row of hinge- 

 teeth, but are separated from it at either end by a short 

 smooth area. Margins provided with such ridges and 

 furrows are said to be cvemdate. 



The adductor muscle-impressions are two in number, 

 anterior and posterior in position. They differ slightly 

 in shape and size, more in some species than in others. 

 From the inner margin a very distinct line runs up to the 

 umbo, marking off the path of shifting of each muscle as 

 the growth of the shell proceeded. 



From one adductor to the other there runs, parallel to 

 the ventral margin, a very distinct line, the pallial line. 

 This takes its name from the mantle or pallium, the two 

 halves of which line the interior of the two valves, much 



