Ml' B. N. Peach on the Occurrence of Pteryyotv.s, etc. 345 



sions which are locally known in Caithness as "Adam's 

 Plates," and which are evidently formed from the posterior 

 segments of Pterygotus like animals vertically compressed. 



Pterygotns Picki, Nov. Spec. — PI. VII., Fig. 1. 



The specimen, for which the name is proposed after the 

 late Kobert Dick, wlio did so much towards bringing to light 

 the denizens of the Lower Old lied Hags of Caitlmess, and 

 from whom in all likelihood John Miller acquired some of 

 his specimens, being a closed ring is one of the last body 

 segments of a large individual. It is quite detached, and no 

 other fragment is found on the same slab with it, though 

 that is a large one. The segment is nearly circular, and 

 measures 5 inches across. From the manner in which it is 

 preserved its depth cannot be made out, though it must have 

 exceeded 3 inches. It is fossilised, end on, with the anterior 

 articulation downwards, and thus entirely hidden in the 

 matrix. Its posterior margin, however, is well exhibited, and 

 as this bears a strongly denticulated fringe which is more 

 pronounced on the dorsal than on the ventral aspect, it thus 

 differs from all as yet described species of Pterygotus. The 

 elongated squames forming the posterior marginal fringe are 

 longest in the mid line of the back. Such a fringe is found 

 on species of the genera Eurypterus and Stylonurus, but as 

 in addition to this the segment bears a raised keel of large 

 squames along the mid line of its dorsum, which losing itself 

 anteriorly is carried back through the hinder half of the seg- 

 ment to its margin, it shows that its owner's affinities are with 

 Pterygotus and with a species allied to the P. Anglicus. This 

 keel, and the fact that the segment is a closed ring, show 

 that we have to deal w^ith one of the last five segments, and 

 in all probability with the antepenultimate segment, i.e., the 

 eleventh abdominal one, or the seventeenth segment, if we 

 consider the cephalothorax as made up of six coalesced seg- 

 ments as in Limulus and scorpion. There seem to have been 

 no lateral crests, and the segment is not depressed, but as 

 already stated, it is almost circular. The test is everywhere 

 ornamented with the characteristic scale-like markings, which 

 are smaller in pattern than on the corresponding segment of 



