188 



PALAEOZOIC TIME — LOWER SILURIAN. 



— (2.) Trilobites* — Fig. 241, Conocephalus minutus Bradley, from Keeseville, 

 N.Y. : a, the head-shield or buckler, with the side-pieces wanting, none having 



Fig. 244 C 



Dalmania Hausmanm. 



* The genera of Trilobites are distinguished mainly by the form and mark- 

 ings of the head and tail portions of the body, and the eyes. The large anterior 

 segment is the head or buckler; the posterior, when shield-shaped and combining 

 two or more segments, the pygidium. The middle area of the head, which is 

 often very convex, is the glabella ; the parts of the head either side of the gla- 

 bella, the cheeks ; a suture running from the anterior side of the eye forward or 

 outward, and from the posterior side of the eye 

 outward (s s in the figure), the facial suture ; a 

 prominent piece on the under surface of the head, 

 covering the mouth, the hypostome. The eyes may 

 be very large, as in Dalmania (fig. 244 C), Phacojys, 

 and Asaphus (fig. 320), or small, as in Homalonotus ; 

 or not at all projecting, as in Trinucleus (fig. 323) ; 

 and may also differ in position in different genera. 



The glabella may be broader anteriorly, as in 

 Phacops, Dalmania, Trinucleus ; or broader pos- 

 teriorly, as in Calymene (fig. 321), Bathyurus (fig. 

 261) ; and it may vary otherwise in form ; or it 

 may be ill defined, as in Isotelus (fig. 320) and Illse- 

 nus (fig. 333). It may have no furrows across its 



surface, or one or more up to four (or rarely five). The four may be numbered, 

 beginning behind, No. 1, 2, 3, 4 (fig. 244 C). These furrows may extend en- 

 tirely across, or be divided at middle as Nos. 2, 3, 4 in fig. 244 C. Isotelus (fig. 

 320) and Illsenus (fig. 333) have none of these furrows; Trinucleus (fig. 323) has 

 No. 1 faint or obsolete; Asap>hus (fig. 332), Homalonotus, and Bathyurus have 

 No. 1 entire ; Dicellocephalus (fig. 242) has Nos. 1 and 2 entire, and 3 divided : 

 Calymene (figs. 177, 242), Dalmania, Gryphcus, Ogygia, Cheirurus, Pro'etus, have 

 No. 1 entire and 2, 3, 4 divided, but 4 is sometimes obsolete. £ao (fig. 261 A) has 

 No. 1 entire and 2, 3, 4 divided, but there is a medial longitudinal depression in 

 which 2, 3, 4 from either side coalesce. In one group, the genus Lichas, the 

 glabella has, on either side, one or two longitudinal or oblique lobes (figs. 322, 

 409). The furrows, as shown in the genus Paradoxides, correspond to articula- 

 tions of the body. They are mostly obliterated in the higher Trilobites where 

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

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



The position of the facial suture (see p. 154 and s s in fig. 244 C) affords cha- 

 racters for distinguishing genera; also the number of segments of the body 

 (in Agnostus the number is very small, and the head and pygidium are almost 

 in contact) ; the continuation of the free movable segments to the posterior ex- 

 tremity, 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. 410) ; 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 

 Calymene, Isotelus, Phacops. 



