234 TRILOBITA CHAP. 



have so far been found in 136 species of Trilobites belonging to 

 39 genera ranging from Lower Cambrian to Carboniferous. 



A " metastoma " or lower lip plate (Fig. 142, Ep) is found just 

 behind the hypostome in Triarthrus, but has not been noticed 

 in any other genus. Between the hypostome and the metastoma 

 lies the mouth. 



The segments of the thorax are free, and their number varies 

 from two in Agnostus (Fig. 146) to twenty-six in Harpes (Fig. 

 150, A). In the Trilobites confined to the Cambrian period the 

 number (except in the Agnostidae) is usually larger than in the 

 genera found in the Ordovician and later periods. Owing to the 

 free thoracic segments many Trilobites were able to curl up some- 

 what after the manner of a Wood-louse (Figs. 137, D, 138), 

 The axial part of each thoracic segment is more or less con- 

 siderably arched. Usually it consists of three parts : (i.) the 

 largest part (Fig. 137, C, a), called the ring, is band-like in 

 form, and is always visible whether the Trilobite is extended or 

 coiled up ; (ii.) in front of the ring is a depressed, groove-like 

 part (Fig. 137, C, Z>) separating it from (iii.) the articular portion 

 (c) which is convex in front and extends beneath the ring of the 

 preceding segment ; this part is only visible when the Trilobite 

 is coiled up or when the segments are separated. In some few 

 genejra the axial part consists of a simple arched band without 

 either a groove or a specially modified articular portion. The 

 pleurae (Fig. 137, A, /, C, d-f) are fixed firmly to the axis, and 

 have the form of narrow bands with the ends rounded, obtuse, 

 pointed, or spinose. In a few cases the pleurae have a plain 

 surface ; but usually they possess either a ridge or a groove 

 (Fig. 137, C, g)\ the former is generally parallel to the margins 

 of the pleura, the latter is generally oblique, being inclined 

 backwards from the axis. Sometimes in front of the ridge 

 there is a small groove. On the ventral surface each pleura 

 shows, at its outer extremity, a reflexed margin or doublure. 

 At some distance from the axis the pleurae are bent downwards 

 and backwards. The point where this bend occurs is called the 

 " fulcrum " (c) ; it divides the pleura into an internal and an 

 external part : the internal part (d-e) is flat or slightly convex, 

 and just touches the front and back margins of the adjacent 

 pleurae ; the external part (e-f) may be (i.) narrower than the 

 internal part, so that it is separated from the previous and 



