UPPER SILURIAN ROCKS OF THE PENTLAND HILLS. 



153 



marked border or selvage, 4 mm. in breadth. The surface of these segments is covered 

 with a very small inconspicuous ornamentation of the usual character. Down the right 

 side of the specimen are seen what I take to be the plate-like abdominal appendages, 

 between which and the body wall traces of branchial leaflets may be seen in the 3d and 

 5th segments. The markings on these abdominal appendages are not shown in this 

 specimen, but in the specimen which shows the central lobe of the genital plate, they are 

 seen to have the form figured in fig. 4, the scales being rather angular, and giving the 

 impression of zigzag lines running across the body. On the dorsal surface (fig. 2) a third 

 type of ornamentation is met with, consisting of very broad (4 mm.) flat scales, in 

 addition to which there is a single row of tubercles along the posterior margin of each 

 segment. This variety of ornamentation in the one region of the body is worth noting 

 as a warning against making species from fragments, the ornamentation on which is the 

 only available character. 



The form of the central lobe of the genital plate is outlined in fig. 5. It differs 

 very markedly from that figured by Dr Woodward in the restoration of S. Logani 

 (Monograph, &c, p. 131), being long (36 mm.) and rounded at the end. Unfortunately 

 the genital plates are not shown. 



The six posterior (metasomatic) segments and the beginning of the telson are only 

 shown in one specimen (fig. 2). The figure represents the cast of the dorsal surface of 

 these segments, certain portions being completed from the other half of the slab. The 

 segments are crushed somewhat obliquely, and this makes the determination of their 

 breadth very difficult, the following figures being only an approximation : — 



Segment. 







Width. 



Length. 



6th, . . 118 mm. 



20 mm 



7th, 







110 „ 



20 „ 



8th, 







94 „ 



23 „ 



9th, 







84 „ 



23 „ 



10th, 







78 „ 



25 „ 



11th, 







60 „ 



27 „ 



12th, 







42 „ 



• ... 



Telson, 







10 „ 



. 



These segments are all produced at the sides into curved " epimeral " pieces, which 

 arise from the posterior corners of the segments. The posterior margins of the segments 

 are ornamented by a single transverse row of slightly elongated tubercles, and the 

 surface is covered, like that of the mesosoma, with large scale-markings. The last 

 segment appears to be very short, with enormously expanded epimerites, and the 

 markings on it are very much smaller than on the preceding ones. 



The Telson is attached between the large epimerites of the last segment, and is 

 12 mm. in width. Unfortunately only a small portion of it is visible, and there is no 

 clue to its probable length. Fig. 8 is one of two fragments of detached telsons which 



