CRUSTACEA. l6/ 



always definite and are almost invariably seven in number. In 

 the meanwhile, therefore, it is safest to regard the Trilobita as 

 a peculiar order, the exact position of which in the Crustacean 

 class is still undetermined. 



The general facts as to the distribution of the Trilobita in 

 past time are soon told. The Trilobites are exclusively Palaeo- 

 zoic, their range extending from the Upper Cambrian to the 

 Lower Carboniferous. If the Palceopyge Ramsayi of the Long- 

 mynd beds be a Trilobite, then the order has its commence- 

 ment in the Lower Cambrian. In the Upper Cambrian Rocks, 

 and especially in the strata which constitute the " Menevian 

 Group " of Salter, and the " Primordial Zone " of Barrande, is 

 a peculiar group of forms commonly spoken of as the " Prim- 

 ordial Trilobites." These belong mostly to the two families 

 Agnostida and Olenidtz, and to the genera Agnostus, Para- 

 doxides, Olenus, Dikdocephalns, Conocoryphe, Angelina, Ellipso- 

 cephalus, &c. 



Many of these forms are distinguished by degraded char- 

 acters, such as the absence of eyes, the want of the facial 

 suture, the segmentation of the glabella, or the multiplication 

 or diminution of the number of body-rings. It should not be 

 omitted to be noticed that the singular tracks which have been 

 described from the Potsdam Sandstone (Upper Cambrian) of 

 Canada, under the names of Protichnites and Climactichnites, 

 are believed with good reason to be the tracks of Trilobites. 

 In the Lower and Upper Silurian Rocks the Trilobites attain 

 their maximum of development, the leading families being the 

 Asaphida, Phacopidce, Trinudeida, Cheiruridtz, and Calymenidce, 

 and the chief genera being Asaphus, Ogygia, Phacops, Trinu- 

 cleus, Ampyx, Cheirurus, Encrinurus, Calymene, and Homalo- 

 notus. In the Devonian Rocks, again, Trilobites are tolerably 

 abundant, though less so than in the preceding series. The 

 commonest Devonian genera are Phacops, Homalonotus, and 

 Brontcus. Lastly, the order seems to die out before the close 

 of the Carboniferous period, being represented in the Carbon- 

 iferous Limestone solely by the three genera, Phillipsia, 

 Brachymetopus, and Griffithides. 



The following gives the leading characters of the more im- 

 portant families of Trilobites, with a few of the chief genera in 

 each family, and the range of the group in time : 



i. AGNOSTID^E. Characterised by their small size, the head 

 and tail being covered by nearly equal and similar shields, and 

 the reduction of the body-rings to two in number (fig. 113). 

 There are no eyes, and the facial suture is wanting. The 

 family includes only one well-marked genus, viz. Agnostus, 



