252 



MINERALS AND GEOLOGY 



with flattened, natatory feet. The Copepods represented by the 

 modern Cyclops, Argulus, &e., have no known fossil-representatives, 

 and the Cladocera are also of very doubtful occurrence in the fossil 

 state. The Phyllopods, represented by the living Apus, Branchipus, 

 Estheria, &c., date apparently from the Devonian period, but no 

 fossil examples have been found, as yet, in Ontario or Quebec. 



4. Trilobita : The Trilobites form an entirely extinct series of 

 Crustacea, related to the Phyllopods on one hand, and to the Mero- 

 stomes on the other. Their remains are found in Cambrian, Silurian, 

 and Devonian strata in great numbers, and sparingly in the Lower 

 Carboniferous beds, above which, no traces of the group have been 

 discovered. The trilobites present a generally oval, tri-lobed 

 form of body, averaging about an inch-and-a-half to three inches in 

 length ; but some examples are scarcely the fourth of an inch, whilst 

 others occasionally shew a length of eight or nine inches. The upper 

 surface of the trilobite was protected by a chitonous or crustaceous 

 shell composed of numerous pieces, in part free, and partly united by 

 sutures. The underside of the body seems to have been covered 

 essentially by a soft or semi-coriaceous integument, and to have 

 carried numerous feet, some of these being "jaw feet" as in the 

 merostomes and copepods, and others probably branchial and natatory 

 in their functions. 



As shewn in the annexed Figure (166), the 

 upper covering or " back " of the trilobite consists 

 of three principal parts: (1) the Buckler or Head- 

 shield, H ; (2) the Body or Thorax, T ; and (3) the 

 Pygidium or Caudal-shield, P. 



The head-shield is always more or less of a cres- 

 cented or horse-shoe shape, with the convex side 

 in front ; and its posterior or so-called genal angles, 

 though rounded in some species, very commonly 

 terminate in points or spines. Its central portion 

 is generally in the form of a distinctly raised 

 area known as the glabella ( = G, Fig. 166). 

 The surface of this is sometimes smooth, but is more commonly 

 lobed, furrowed, or granulated. In certain genera ( Phacops, &c.) the 

 glabella is enlarged or expanded anteriorly, and in others (Calymene, 



FIG. 166. 



