1852.] SALTER ON PTERYGOTUS PROBLEMATICUS. 387 



5 lines broad, and has one of its edges closely serrated. It is of the 

 same thin substance as the other fragments, and is striated longitu- 

 dinally, like the other spines, the strise radiating upwards from its 

 base. It is perhaps the terminal joint of one of the feet, or, if arti- 

 culated with the fragment to which it is attached, it might indicate 

 one of the lateral appendages such as are found in the abdomen of 

 LwmluSy and the convex spiny border might then well be part of 

 the abdomen itself. This, I am inclined to think, is the right 

 explanation. 



In the collection of the Geological Survey there are fragments of 

 the limbs of this Crustacean, found in the ^ Tilestones ' at the base 

 of the Old Red Sandstone at Kington, Presteign. Two or three 

 joints, each about an inch long, and the broadest more than half an 

 inch in width, are found together ; they are compressed toward their 

 inner edges, and thickened on the back or outer edge, which is also 

 marked by several of the semicircular folds so characteristic of the 

 entire crust. With these is associated a large fragment like the tail- 

 flap of a lobster (but much more probably the base of one of the 

 thoracic feet), of which a fragment measures 2\ inches ; it is very 

 thick on one side, and towards its outer margin is covered by closely 

 set tubercles, which become small spines on the edge itself. The 

 thick portion is smooth externally, or has only a few of the charac- 

 teristic folds ; and the outer edge shows no trace of the large crowded 

 spines or appendages so conspicuous in this portion of the P. an- 

 glicus. 



It is probable that there are numerous species of the genus in the 

 old rocks. Fragments with the characteristic markings occur in 

 Upper Silurian strata at Gaspe, Lower Canada ; and portions of the 

 limbs of a Bohemian species have been figured by the late M. Corda 

 as the feet of Brontes *, a genus of Trilobites. 



Naturalists seem to be agreed that the Pterygotus was a gigantic 

 Entomostracan, and Agassiz published it as such, in the explanatory- 

 note to his figures. It differed from Limulus in having the segments 

 of the abdomen freely articulating with each other t> in this respect 

 agreeing with Eurypterus, a genus of equally gigantic Crustaceans, 

 which Prof. M'Coy, I think very happily, arranges in the Fcecilopoda. 

 He has examined perfect Scotch specimens of Pterygotus, and tells 

 me that "the eye-like pits on the shield are very like those oi Eury- 

 pterus, but they are as large as the orbits of a horse's eye." I hope 

 he may be induced to present his notes on this genus and his drawings 

 to the Society. 



* Prodr. Trilob. Sil. Syst. Boheme, pi. 4. f. 33. 



t It may be worth while here to remark, as there seems to be a misconception 

 about it on the part of some foreign naturalists, that the Limuli of the Coal-for- 

 mation {Bellinurus, Konig) differ in nothing from the true Limuli except in the 

 points of the anchylosed abdominal segments being more produced ; they are by 

 no means separate or movable, as numerous specimens show. The eyes too, 

 though not well shown in Mr. Prestwich's figures (Trans. Geol. Soc. 2 Ser. vol. v. 

 pi. 41), are in the usual position in all the species. 



