86 BRITISH FOSSIL CRUSTACEA. 



are characteristic of the beds of passage above the top of the Ludlow series. P. problem- 

 aticus may now, therefore, be considered an established species, and the cabinets of our 

 Ludlow friends, Messrs. Cocking and Marston, have furnished many of the materials. It 

 occurs, too, in plenty, as Mr. Lightbody's researches show, in the transition beds beneath 

 the Old Red Sandstone (pi. xiv, op. cit.) ; and Mr. J. Harley, to whom we are indebted 

 for much valuable help, has been fortunate enough to detect its fragments far up in the 

 Cornstones of the Old Red itself, a higher limit than the genus had been known before 

 to attain. 



[If the large fragments from the transition beds above quoted, and figured on pi. 

 xiv, Mem. Geol. Surv., Mon. I, be of the same species as the sculpture indicates, the 

 body-segments attained a very large size, nearly three inches from back to front. As it is 

 possible these may belong to a different species, I will describe these portions first. 



Carapace (?) (pi. xiv, fig. 16, Mem. Geol. Surv., Mon. I). — A fragment, three inches by 

 two and a half, has the surface sculptured, unlike the body-rings, i. e. much more finely 

 marked, and without the regular increase in size and curve of the plicae backwards. The 

 anterior ones are nearly as much bent as the hinder ones, though smaller ; all are but slightly 

 prominent, and are covered by numerous smaller plicae. 



Body-rings (pi. xiv, fig. 17, op. cit.). — One of the broad abdominal rings, two inches 

 and three quarters from back to front ; the articulating front margin is rather deeply con- 

 cave, and its edge is obscurely striated longitudinally. The plicae are very numerous 

 and close-set, not so large as in P. anglicus, or so straight on the forward edge, where, 

 however, they are very closely packed. They are more open posteriorly, and cover more 

 than half the segment, interspersed with very numerous minute semicircular plaits. 



Another piece (fig. 14, op. cit.) is here considered as belonging to this species, but 

 it only shows the interspersed plicae over part of the surface, and it quite possibly belongs 

 to P. ludensis, or even to a new species. 



Penultimate Joint (pi. xiv, fig. 18, op. cit.) — Of this we have only the lower 

 surface.; and as the plicae are restricted to the upper portions, and only a few small ones 

 are interspersed, it is possibly not P. problematicus, but of the same species as the one 

 (fig. 15) mentioned above. The width is greater than that of the same joint in P. anglicus 

 or P. ludensis, being three inches and a half, while the length is only two and a quarter (or 

 as fourteen to nine), which is about the proportion in P.gigas. The joint is not expanded 

 posteriorly, as in that species, and the plicae are semicircular, not pointed, on the 

 lower side.] 



In the true Ludlow Rock but few body-joints have been met with ; the two best are 

 figured from Mr. J. Harley's collection, viz. pi. xii, fig. 20, op. cit., must be one of the 

 thoracic" rings, and fig. 21 probably the tenth or last but two of the segments. Both 

 show the minute interspersed plicae very clearly, and these small plicae extend over nearly 

 all the segment, while the larger ones are confined to the anterior half. 



Telson as yet unknown, as also is the thoracic plate. 



