178 EVIDENCE OF THE SILURIAN M0LLUS63. 



such as the lancelet, want, — a brain enclosed in a cartilagi- 

 nous cavity in the head, and perfectly formed organs of sight ; 

 they possess, too, what is found in no other mollusc, — organs 

 of hearing ; and in sagacity and activity they prove more than 

 matches for the smaller fishes, many of which they overmas- 

 ter and devour. With this highest class there contrasts an 

 exceedingly low molluscous class at the bottom of the scale, 

 or, at least, at what is now the bottom of the scale ; for they 

 constitute Cuvier's fifth class ; while his sixth and last, the 

 Cirrhopodes, has been since withdrawn from the molluscs 

 altogether, and placed in a different division of the animal 

 kingdom. And this low class, the Brachiopods, are creatures 

 that, living in bivalve shells, unfurnished with spring hinges 

 to throw them open, and always fast anchored to the same 

 spot, can but thrust forth, through the interstitial chinks of 

 their prison-houses, spiral arms, covered with cilia, and win- 

 now the water for a living. Now, it so happens that the 

 molluscan group of the Silurian system is composed chiefiy 

 of these two extreme classes. It contains some of the other 

 forms ; but they are few in number, and give no character 

 to the rocks in which they occur. There was nothing by 

 which I was more impressed, in a visit to a Silurian region, 

 than that in its ancient graveyards, as in those of the present 

 day, though in a different sense, the high and the low should 

 so invariably meet together. It is, however, not impossible 

 that, in even the present state of things, a similar union of 

 the extreme forms of the marine mollusca may be taking place 

 in deep-sea deposits. Most of the intermediate forms pro- 

 vided with shells capable of preservation, such as the shelled 

 Gasteropoda and the Conchifers, are either littoral, or restricted 

 to comparatively small depths ; whereas the Brachiopoda are 

 deep-sea shells ; and the Cephalopoda may be found voyaging 

 far from land, in the upper strata of the sea above them. Even 

 in the seas that surround our own island, the Brachiopodou?" 



