178 



AFFINITIES AND GEOGEAPHICAL 



or coriaceous integument, whence their name. It thus appears 

 that the JBrackiopoda, which are the predominant fossils of the 

 lower Silurian era, are the first animals we meet with in this line, 

 having parts capable of commemorating their existence. While 



FIG. 92. 



B 



A, Terebratula reiiciilaris ; B, Interior of Spirifer hystericus. 



the Brachiopoda are generally inhabitants of deep seas, the 

 Lamellibranchiata, among which are included the oyster, mussel, 

 and other testacea, affect the beds of shallow seas, whence they 



spread in a variety of genera, 

 towards shores, the mouths of 



FIG. 93. 



rivers, and into fresh water. 



The Lamellibranchiates are 

 higher than the preceding 

 class ; they are the first bi- 

 valves which possess a true 

 hinge (Fig. 93). It is also re- 

 markable that, with the decline 

 of the brachiopods, at an early 

 point in the secondary forma- 

 tion, rises the lamellibranchiate 

 family. There is here, there- 

 fore, an improvement in orga- 

 nization, an advance in habitat 

 landward, and a succession of 

 existence in the geological ages, 

 all in harmonious connexion. 

 Nor is this all. The lamel- 

 libranchiata are again divisible 

 into monomyaria and dimyaria, the former having one adductor 

 muscle, and the latter two ; the former, moreover, being inter- 

 mediate between the brachiopods and dimyaria in respect of 



Cytherea concentrica. 



