UPPER SILURIAN ROCKS OF THE PENTLAND HILLS. 



155 



shoe-shaped, but does not narrow towards the posterior margin so markedly as in S. 

 ornatus. The breadth at the widest portion, which is close to the front margin, is 40 mm., 

 and at the posterior margin 35 mm. The margin is bounded in front by a narrow border 

 (2 mm. wide), which runs out about half way down. A well developed ridge runs down the 

 centre to within about 1 cm. of the front margin, and on each side of this are placed the 

 prominences for the lateral eyes. These prominences are about half as long as the 

 carapace (16 mm.), and about 10 mm. wide. The eye itself runs as a curved band, 

 3 mm. broad, round the anterior and outer sides of the prominence. No trace of the 

 occelli can be seen. The surface of the carapace is covered with a well marked, strongly 

 curved scale ornamentation, and the posterior border is marked by a marginal row of 

 elongated tubercles. 



The dimensions of the body segments are as follows : — 



1st, 



34 mm. wide. 



6 mm. long 



2nd, 



34 . 



6 



3rd, 



38 . 



8 



4th, 



38 



6 „ 



5th, 



38 



6 „ 



6th, 



. 32 „ 



6 „ 



7th, . 



30 



8 „ 



8th„ 



.. 27 „ 



9-5 „ 



9th, 



24 



10 



10th, 



• 22 „ 



11 „ 



11th, 



. 17? „ 



11-5 ,„ 



12th, 





. ... 



The segments thus diminish in width from the third, and increase markedly in length 

 from the 7th on. The anterior segments show, a scale ornamentation very similar to 

 that on the back of S. ornatus, but confined chiefly to the front portion of each segment. 

 The posterior margin of each segment is marked by a row of tubercles, which are particu- 

 larly conspicuous in the posterior segments. The last segment, and probably those 

 preceding it, had epimera. 



The Telson (fig. 11) is not less than 52 mm. in length, but the point is unfortunately 

 lost. It is 15 mm. wide at its point of attachment, and rapidly narrows to 5 mm., 

 beyond which it tapers very gradually. It is deeply grooved by a pair of longitudinal 

 furrows, and the median ridge between them is marked, by faint oblique denticula- 

 tions. 



The Metastoma, the outline of which can be made out through the carapace (fig. 10), 

 is 10 mm. wide at the posterior margin, which is straight, and becomes rapidly narrower 

 towards the front. The front end of it is not visible. 



The limbs are only partly shown, but are very characteristic. On the right side 

 (fig. 10) segments of the posterior limb are shown. The limb is very broad and short in 



