A BEVIEW OF THE TEILOBITES. 279 



are found, however, and not unfrequently, in limestone deposits, asso- 

 ciated with brachiopods and other forms usually referred to deep- 

 sea types. But the brachiopods are now well-known to range from 

 extreme depths up to the very tide-line : and hence their presence 

 in trilobitic rocks, does not speak against the littoral origin of such 

 deposits. In many instances, it is evident, that the palaeozoic lime- 

 stones, as those of other ages, were derived more or less directly 

 from coral reefs ; and these reefs may have afforded shelter to many 

 trilobites. Along the inner edge of the great barrier reef of north- 

 eastern Australia for example, where in many places a depth of no 

 more than ten or twelve fathoms exists, different species of both 

 brachiopods and crustaceans are often met with. 



"Whilst some observers imagine the trilobites to have been more or 

 less sedentary, others contend that they must have been in constant 

 motion — swimming, back downwards, it or near the surface of the 

 sea. The truth lies probably between the two. As already pointed 

 out, the presence of eyes is a strong argument against a sedentary 

 existence, and the rolled up condition of body (so commonly witness- 

 ed) speaks equally, on the other hand, against a state of constant 

 motion. It is difficult to conceive that these extinct forms could have 

 been endowed with strong swimming powers, for no traces, even un- 

 der the most favorable circumstances for preservation, of floating- 

 appendages have been met with ; and their branchial feet, allowing 

 such to have been present, could not have constituted swimming or- 

 gans of any force. The unequally balanced extremities of many 

 species, although in part perhaps compensated by the downward ex- 

 tension of the genal angles of the head-shield, is also an obstacle to 

 the satisfactory adoption of this view. At the same time, it should 

 be observed, that their shell, from its general thinness, must have 

 been comparatively light ; and the flattened form of body conducive 

 to a certain degree of buoyancy. A slight movement of the flexible 

 thorax and caudal extremity probably formed a sufficient propelling 

 power for the animal's wants. When alarmed, the contraction of the 

 body would enable it to sink with ease into deeper water ; and in its 

 power of adhering by its under side to rocks and to the sea-bottom 

 generally, it possessed a further means of defence against its enemies. 

 By this power of adhesion, moreover, individuals may have been car- 

 ried on floating bodies over a wide range of coast or acro&s open seas, 

 and thus have given rise to colonies in localities far distant from 

 their normal centres. In this manner the extended geographica 

 limits of certain species may perhaps be accounted for. 



