58 Fossils of the Niagara and Clinton Grouj^s. 



its length from the central lobe of the body outwards ; the lateral segments 

 of the tail are also grooved, and terminate in a thickened continuous mar- 

 gin which borders the whole of the posterior edge, and is extended into the 

 terminal spine. The tail, pygidium, or caudal shield, as it is variously 

 called, consists in the trilobites of only one piece, and what appears to be its 

 division into segments are only furrows in its surface, arranged in the 

 dii'ection of the articulations of the body. In a recent large work upon the 

 trilobites of Bohemia, [Systenie Silu7'ien de la BoJieme) its Editor, M. 

 Barrande, shews that the young animals have but two or three segments in 

 their body, and that as they become older others are developed out of the 

 caudal shield. The front part of the shield is first furrowed across, and in 

 course of time this furrow deepens, until it finally cuts off a new segment, 

 which thereafter belongs to the body. One segment after another thua 

 separates itself from the tail, until the animal has attained the number of 

 the adult individual. In many genera of trilobites, such as Calymene and 

 Phacops, the furrows upon the pygidium appear to mark out so many seg- 

 jnents of the body, which never become completely developed. In othera, 

 Buch as Isotelus and Illsenus, the tails are smooth, and not at all, or only 

 very slightly furrowed. 



The eyes of this species are of a crescent form, with the convex curve 

 ■outwards, and they are on this side, the outside, covered with numerous 

 «mall lenses. The structure of this organ is thus compound, like that of 

 certain insects. In Phacops caudatus, the English species, there are 

 ^' about 240" in each eye,- and it is probable that the American species has 

 near the same number. This is the most abundant trilobite in the upper 

 Silurian rocks of America. In Hall's Paleontology, it is called a Phacops, 

 but in the more recent classification, adopted by JNIr. Barrande, in the work 

 above quoted, that genus is divided into two, Phacops and Dalmanites. 

 It is in the latter genus that our species will most probably be classed here- 

 after. 



Phacops from the Greek Phakos, a " lens ;" and Ops, the "eye." — 

 The specific name is probably from Limulus, the " king arab," and Oura, 

 a " tail ," this trilobite having a tail like that of the king crab, 



Pentamerus oblongus is a fossil shell peculiar to the Clinton group, 

 and of a very variable form. It is generally of an oblong oval shape, with 

 a surface either smooth or but slightly marked by faint concentric lines. In 

 old full grown shells there are several concentric ridges, indicatiuj? stages 

 of growth. Professor Hall says : " In the smaller and medium sized forms, ■ 

 the shell has a general oval or ovate form, sometimes slightly trilobate at 

 base, it is so much depressed, that the thickness or depth of botli valves ia 

 only abo'at half the width. This proportion sometimes continu3s even in 

 very old sh3ll5, the trilobate chxracte- of the ba=5e bein]' of.:en very conspic- 

 uous. In the majority of the specimens, however, the valves become 

 gradually more gibbous as the shell increases in size, and the trilobate 



^ This is the number given by Mr. Salter, in the 2nd Decade of the Geological 

 Survey of England, and he states that tiie number ^00, given in Buckland'* 

 Bridge water Treatise, was probably intended for both eyes 



