LOWER SILURIAN. 



175 



well as in the following. Fig. 251 represents one of the largest kinds, 

 a species of Paradoxides, that at times 



exceeded twenty inches in length. It Figs. 251-253. 



is from the beds near Braintree,Mass. 

 Fig. 252 represents the cephalic 

 shield of another Trilobite, of the 

 genus Conocoryphe, from St. John ; 

 and Fig. 253, the cephalic and caudal 

 (head and tail) portions of another 

 genus, Agnostus. These three gen- 

 era of Trilobites have many species 

 in Primordial rocks, and mark this 

 era in the history of life. 



In the rocks of the Potsdam epoch, 

 various fossil sponges are found (Fig. 

 261, p. 177) ; remains of Crinoids ; 

 Brachiopods of the genera Lingu- 

 Mla (Figs. 262, 263, 264), Ortkis, 

 etc.; and various Trilobites (Figs. 266 

 —269), but among them none that 

 were alive in the Acadian epoch, and 

 none of the genus Paradoxides. 

 Nearly 100 species of Trilobites have 

 been described from the American 

 Primordial rocks. 



There are also the first of Grapto- 

 lites, delicate plume-like fossils, so 

 named from the Greek ypa^oj, / write. 

 They are described as Hydroid Aca- 

 lephs on page 130. Fig. 270 repre- 

 sents one species, natural size, and 

 Fig. 271, a portion of a branch en- 

 larged : it is from the Wisconsin beds. 



Trilobites. Fig. 251, Paradoxides Harlani 

 (XX); 252, Conocoryphe Matthewi; 

 253, Agnostus Acadica — a, head, b, cau- 

 dal part. 



oblique lobes (Figs. 362, 449). The furrows, as shown in the genus Paradoxides, cor- 

 respond to articulations of the body. They are mostly obliterated in the higher Trilo- 

 bites where the head-shield is most compact, and are most distinct in the lowest, like 

 Paradoxides, being a part of that general looseness of body that marks inferior grade. 



The position of the facial suture (see p. 174 and s s in Fig. 254) affords characters for 

 distinguishing genera; also the number of segments of the body (in Agnostus, Fig. 279, 

 the number is very small, and the head and pygidium are almost in contact); the con- 

 tinuation of the free movable segments to the posterior extremity, or the union of the 

 posterior into a shield (called the pygidium); in some cases the breadth of the middle 

 lobe of the body as compared with the lateral, it being very broad in Homalonotus (Fig. 

 450); the form of the fold of the shell beneath the head at its anterior margin; the 

 shape of the hypostome ; the capability of folding into a ball by bringing the abdomen 

 •to the head, as in Cahjmene, Isotelus, Phacops. 



