524 



CRUSTACEA. 



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 hirsutd), the 

 most minute larvae observed 

 possess a head-shield, but have 

 no pygidium, while the thorax is 

 either wanting or rudimentary. 

 In another type (Agnostus), the 

 larva has both the head-shield 

 and pygidium in a developed 

 condition, 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 incom- 

 plete ; and in a fourth group, 

 though the thorax possesses 

 the number of rings proper to 

 the adult, the pygidium is im- 

 perfect. 



Fig- 379- — Restoration of the under surface ot 

 Calymene Blumenbachii, showing the appendages. 

 d, Doublure of the cephalic shield; k, Hypos- 

 tome ; b, The last pair of cephalic appendages 

 (maxillipedes); m, Mouth, bounded by the jaws; 

 a, Thoracic limbs ; a', Abdominal limbs. (After 

 Walcott.) 



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 may perhaps have been pro- 





A 



c 



Fig. 380. — A, Tangential section of the crust of Calymene senaria, showing the openings of 

 large and small tubes ; b, Vertical section of the same ; c, Tangential section of the crust of 

 Asaphus Canadensis, showing a minutely porous structure. All the figures are greatly magnified. 

 (Original.) 



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

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

 tion are those which have been described by Professor Owen from the 

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



