360 ARTHROPODA. 



a twenty-fifth to a fiftieth of an inch in diameter, and they 

 seem to have been deposited in clusters. The larval con- 

 dition of the Trilobites is only known in certain cases, and 

 it is possible that the young may often have been naked. 

 This subject has been chiefly worked out by Barrande, who 

 has shown, that, so far as our present knowledge goes, the 

 development of the Trilobites follows one or other of four 

 principal lines. In one group of forms (e.g., Sao hirsuta), 

 the most minute larvae observed possess a head-shield, but 

 have no pygldium, and the thorax is either wanting or 

 rudimentary. In another type (Agnostics), the larva has 

 both the head -shield and pygidium in a developed con- 

 dition, but the thorax is wanting. In a third, all the three 

 regions of the body are present, but the thorax and pygidium 

 are at first incomplete ; and in a fourth group, though the 

 thorax possesses the number of rings proper to the adult, the 

 pygidium is imperfect. 



As to their mode of life, the Trilobites, as before remarked, seem to 

 have delighted in muddy bottoms, though often found in limestones, and 

 they must have frequented particular localities in vast numbers. In 

 connection with this subject, we may briefly notice certain tracks or 

 markings in the rocks, which have been supposed to have been pro- 

 duced by these extraordinary extinct Crustaceans, or by their allies the 

 King-crabs and Eurypterids. The most interesting of the tracks in 

 question are those which have been described by Professor Owen from 

 the Potsdam Sandstone (Upper Cambrian) of Canada, under the name of 

 Protichnites. The tracks upon which this genus is founded (fig. 217, 

 A and B) consist essentially of a median groove, with a number of de- 

 pressions or footprints on each side in corresponding pairs, these being 

 often arranged in answering groups, of seven or eight pairs each. Some- 

 times the pits or footprints are replaced by shallow grooves, on each side of 

 the main median groove (fig. 217, B). The tracks of Protichnites indicate 

 the existence in the Upper Cambrian of some animal of very considerable 

 size, since they are sometimes half a foot or more in width. That the 

 animal producing these tracks was a Crustacean can hardly be doubted ; 

 the median groove being made by the tail-spine, and the lateral mark- 

 ings by the feet ; and as we know that large Trilobites actually lived 

 during this period, it seems most reasonable to suppose that we have in 

 these the real makers of the tracks. Principal Dawson, however, has 

 shown that tracks of a closely similar nature are formed by the living 

 King-crabs (Limulus), and he would therefore ascribe Protichnites 

 rather to the Eurypterids. The same eminent observer has also shown 

 that smaller forms of Protichnites occur in the Carboniferous; and he 



