PLEITROGRAPTUS. 1 1 9 



Genus PLEUROGRAPTUS, Nicholson. 



1867. Pleurograpsus, Nicholson, Geol. Mag-., vol. iv, p. 257. 



Polypary bilaterally symmetrica], consisting of two miiserial main stipes, which 



diverge from the sicula at angles slightly exceeding 180°, and from one 



margin (or both) of which numerous, usually irregularly disposed, 



secondary uniserial branches are given off. These secondary branches 



may be simple or compound. 



Thecse of the type of Leptograptus. 



The whole appearance of the Pleurograptus polypary is characteristically slender, 



though its dimensions vary with the age and size of the individual. The sicula is 



but rarely well preserved, though its position is usually clearly indicated; from it or 



its immediate neighbourhood, as in the Leptogrdpti, a central stipe or branch is 



frequently developed, but in Pleurograptus this branch appears to resemble more 



closely the secondary branches given off from the main stipes, rather than the main 



stipes themselves. The secondary branches resemble in most particulars the main 



stipes from which they arise, and the same is also true of the tertiary branches; but 



while the main stipes of the polypary increase gradually in breadth throughout 



their length, the width of the secondary and tertiary brandies is approximately 



uniform, and is the same as that of the main stipe at the point where the secondary 



branches are given off. 



Systematically, Pleurograptus is best regarded as a Leptograptus, from the main 

 stipes of which numerous simple or compound secondary branches are given off at 

 irregular intervals. One species only is known at present, P. linearis, and this 

 shows great variation in the sweep of the main stipes, and in the number and 

 distribution of the secondary branches. One of these variations appears to be more 

 or less permanent, and as it is not an uncommon fossil, we have distinguished it as 

 var. simplex. 



Pleurograptus linearis (Carruthers). Plate XVI, fig. 7; Plate XVII, fig. 1. 



1858. Cladograpsus linearis, Carruthers, Trans. Roy. Phys. Soe. Edinburgh, p. 407, fig. 1. 



1859. Cladograpsus linearis, Carruthers, Ann. Mag. Nat. Hist. [3], vol. iii, p. 24, fig. 3. 

 1867. Dendrograpsus linearis, Carruthers, Geol. Mag., vol. iv, p. 70. 



1807. Pleurograpsus linearis, Nicholson, Geol Mag., vol. iv, p. 257, pi. xv, figs. 1 — 5. 

 1876. Pleurograptus linearis, Lapworth, Cat. West. Scott. Foss., pi. iii, fig. 69. 



Main stipes of great but unknown length, widening gradually and persistently 

 from their origin to a maximum breadth of about 1 mm. Secondary and 



Hi 



