52 PALEONTOLOGY 



taxodont. As there are two adductor muscles, the shell is 

 dimyarian ; and, as these two are approximately equal in 

 size, it is isomyarian. As there is no pallial sinus, it is 

 integripalliate. 



2. Nucula is a genus represented by several hundred 

 species ranging from Silurian to Recent, and varying 

 very little in that long period. The following description 

 applies specially to the Pliocene and Recent species, 

 N. nucleus, but in all essentials it will apply to any other. 

 (Fig. 13). 



The shell is oval tending to triangular, equivalve, the 

 surface (as preserved in the fossil state) nearly smooth 

 with very delicate concentric striae. The umbones are 

 opisthogyral, and situated much nearer the posterior 

 end, so that without a knowledge of the living animal 

 the posterior end would be taken for anterior. There is 

 nothing like the large dorsal area of Ptctunculus, but the 

 region just behind the umbo is slightly flattened or con- 

 cave, and in some species forms a large depressed area 

 called an escutcheon. Internally, the hinge is taxodont, there 

 being a narrow hinge-plate bearing a long row of simple, 

 slightly curved teeth. Just below the umbo this row is 

 bent and almost interrupted, and the hinge-plate extends 

 downwards in a projection that bears a deep hollow 

 facing its fellow in the other valve. These hollows are 

 ligament-pits, the representative of the elastic ligament 

 running across the median plane from one to the other. 

 Such a ligament, lying below the hinge-line, is described 

 as an internal ligament, or better as a resilium, for when the 

 valves are closed it is under compression, not under 



