PTERYGOTUS PROBLEMATICUS. 85 



This post-oral plate, in its anterior portion, a good deal resembles the fragment figured 

 in pi. ix (op. cit.), the notch being a little less deep only. The shape is much more 

 elongate than in P. anglicus, the length being as seven to four ; the width is greatest at 

 the anterior third, and the general shape ovate. The plicae are large and open, and are 

 confined to the anterior portion about the notch. 



The basal joint of the swimming-foot (fig. 8), found at the same locality, is in almost 

 every respect like that of P. gigas, having the teeth broad and short. 



Locality. — Downton Sandstone (Uppermost Ludlow Rock) of Kington, Hereford- 

 shire. (Cabinet of Mr. R. Banks, of that place.) Some specimens, presented by that 

 gentleman, are in the Museum of Pract. Geology ; as are also specimens collected by Mr. 

 A. Marston, of Ludlow. They were found at the Ludlow Railway Bridge, in the passage 

 beds at the base of the Old Red Sandstone. 



Species 8.— PTERYGOTUS PROBLEMATICUS :— Salter. 



Pteeygotus problematicus, Agassiz. In Sil. Syst., 1839, p. 606, pi. iv, figs. 4 and 5 



(and 6, tooth of Sphagodus pristodontus, Ag., 

 tooth only). 



Strickland and Salter. Quart. Journ. Geol. Soc, 1852, vol. 



viii, pi. xxi, figs. 1, 2, p. 386. 



Siluria, 2ud edit., 1856, pi. xix, figs. 4 — 6. 



„ 3rd ed., 1859, „ p. 265. 



„ 4th ed., 1867, „ p. 237. 



Salter. Mem. Geol. Surv., 1859, Mon. I, p. 89, pi. xii, figs. 



7—16, 20,21 ; and pi. xiv, figs. 16—18. 



As this is the principal, if not the only, species in the true Upper Ludlow Rock which 

 has the usual semicircular ornamental plicae, it is to this that the name problematicus 

 should be given ; and fortunately, on one of the minute original fragments figured in the 

 ' Silurian System/ the small intermediate plicae are to be seen marking the species more 

 definitely. 



The large chela, figured as above under this name, by the late Mr. Strickland and 

 myself, proves to be really an appendage of this same species, at least it is always 

 associated with it in the same bed. Again, the antennary portions, fragments of body- 

 rings, bases of the swimming-feet, post-oral plate, &c. (pi. xii, Mem. Geol. Surv., 

 Mon. I), are all found in the Whitcliffe, Ludlow, or other localities of the Upper Ludlow 

 Rock, and clearly differ from the corresponding parts in P. (now Eurypterus) punctatus, 

 the only other large species occurring with them, as well as from those just described which 



