17l' 



and has one of its edges closely serrated. It is of the same thin substance as 

 the other fragments, and is striated longitudinally like the other spines, the 

 striie radiating upwards from its base. It is perhaps the terminal joint of 

 one of the feet, or, if articulated with the fragment to which it is attached, 

 if might indicate one of the lateral appendages such as are found in the ab- 

 domen of Stimulus, and the convex spiny border might then well be part of 

 the abdomen itself. This is probably 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. 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 towards their inner edges, and thickened on the back or outer 

 edge, whj^i 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. 

 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 por- 

 tion is smooth externally, or has only a few of the characteristic folds ; and 

 the outer edge shows no trace of the large crowded spines or appendages so 

 conspicuous in this portion of the Pterygotus Anglicus. 



Naturalists seem to be agreed that the Pterygotus was a gigantic Ento- 

 mostracan, and Agassiz published it as such. It differed from Limulus in 

 having the segments of the abdomen freely articulating with each other. In 

 this respect it agrees with Eurypterus, a genus of equally gigantic crustaceans, 

 which Professor McCoy happily arranges in the Peecilopoda. He examined 

 perfect Scotch specimens of Pterygotus, and found the eye-like pits oa the 

 shield very like those of Eurypterus, but they are as large as the orbits of a 

 horse's eye. 



It is probable that there are numerous species of this genus yet to be 

 found in the old rocks. 



