196 E. T. Netdon — GraptoUten froui Pent. 



The whole mass of shale seems to be full of these graptolites, for, 

 wherever split open, many white examples are displaj'ed upon the 

 surface of the black shale (Fig. 1), all of which are referable to the 

 genus Diplograptiis, and although differing somewhat in width thej' 

 are so similar in other respects that they can hardly represent more 

 than one species. The longest and most perfect specimen (Fig. 2) 

 does not exceed 25 mm. in length and 3 mm. in width at the widest 

 part; but this example has no projecting virgula, such as is seen on 

 other specimens extending perhaps 4 or 5 mm. beyond the thecce. 

 Some examples are a little narrower, while one is as much as 5 mm. 

 wide. The polypary has a small radicle and two cornua at its 

 proximal end, and thence increases somewhat rapidly in width for 

 about a third of its length and then decreases slightly and very 

 gradually to the distal extremity, from which, in several specimens, 

 a virgula extends. In one instance the virgula may be traced 

 throughout the length of the polypary. The theca3 diverge from 

 each side of the axis at an angle of rather less than 45° ; the 

 apertural margin is nearly at right angles with the axis, and the 

 outer free margin is in most cases slightly convex; but there is some 

 variation in all these particulars, even in the same polypary. There 

 are 11-13 thecas in 10 mm. These Peruvian Diplograptids very 

 closely resemble the D. truncatus of Lapworth,^ and Professor 

 Lapworth, who saw the specimens for a few minutes, was good 

 enough to point out this near resemblance. The small differences 

 which may be noticed, namely, the distinct virgula, the somewhat 

 smaller thecte, and the less oblique apertural margin, as well 

 apparently as the shorter polypary, probably indicate that specific 

 difference which one is led to expect from the widely separated 

 habitats of the two foi'ms ; at the same time one hesitates to give 

 them a new name, and would prefer to record them as Diplograptiis, 

 cf. truncatus, Lapw., and as probably of Bala age. 



Little is known of the geology of the immediate area from which 

 Mr. Jessop obtained his graptolites; but David Forbes, in his paper on 

 "The Geology of Bolivia and Southern Peru,"- not only gives a large 

 area of Silurian rocks extending from the south-east to the north-west 

 border of his map, which is perhaps within a hundred miles of Monte 

 Bello, but says that these Silurian strata extend as far as Cuzco, and 

 this would be as far north and well to the west of the district now 

 in question. David Forbes does not aj)pear to have visited this area 

 himself, and the fossils collected further south, which were described 

 by J. W. Salter, were said to indicate beds of Upper Silurian age, and 

 probably Lower Silurian also. The fossils doubtfully referred to 

 the Lower Silurian certainly left much to be desired. No graptolites 

 were found, and consequently a most important guide to the age of 

 these old rocks was wanting. 



D'Orbigny, during his travels in Bolivia,^ found certain graptolites 

 at Tacopaya, near the Eio Grand (lat. 19° S. ; long. 63° 40' W.), 



1 Proc. Belfast Nat. Field Club, ser. ii, vol. i, pt. 4, Appendix, p. 133, 1876-7. 



2 Quart. Joui-n. Geol. Soc, vol. xvii (1861), p. 53. 



•^ "Voyage dans I'Ameriqxie Meridionale " : Pala^out., vol. iv (1842), p. 23. 



