80 BRITISH TOSSIL CRUSTACEA. 



fragments were all considered to belong to the P. problematicus of Agassiz, a species for 

 which there is unfortunately very scanty material, but which, as originally described, is a 

 Ludlow Rock species identical with one of the Kington fossils, but not apparently with 

 the principal and largest of them, here described, and which in many respects is very 

 like P. anglicus. 



P. gigas has, in common with the latter species, the open scale-like sculpture on the 

 body-rings, and the thick tubercular scales on their margin ; the shape of the epistome 1 

 and head is very similar, but the latter is rounded, and not truncate in front. 2 The 

 penultimate body-ring is wider and has a short keel on the upper surface only (while 

 P. anglicus has one on both sides above and below), 3 and the tail-joint or telson appears 

 to be emarginate instead of pointed. But if this character should be deceptive, there 

 can be no doubt of the specific difference, since this joint is furnished with a most 

 remarkable elevated crest or ridge, nearly half an inch high, which is quite absent in the 

 Scotch species.* 



As the fragments indicate a species of the largest size, the above specific name will 

 not be inappropriate. The size of the chelate antennas exceeds that of any known species. 

 There are two species in the Kington beds, P. gigas and P. problematicus, and it was, 

 of course, possible that some of the parts assigned to the former might belong to the 

 latter species. The subsequent discovery of nearly all the parts of P . problematicus shows 

 that in this instance the fragments have been rightly collocated. 



Head qx Carapace (pi. viii, fig. 1, Mem. Geol. Surv., Mon. I). — Nearly semi-oval, con- 

 vex, the width three inches and three quarters at the broad base, probably greater than the 

 length. The specimen being imperfect behind, the true length is not known, but the 

 portion preserved is three inches and a quarter long. The eyes are very large, three 

 quarters of an inch long, oval, and prominent beyond the margin. They are placed very 

 near the anterior end, and the space between them on the margin is about an inch and 

 three quarters, while between the convex inner edges of the eyes it is about an inch and 

 a half. The anterior border is arched and very slightly angular in front, with a crenulate 

 edge. The sides are convex, their margin not visible. (See Woodcut, fig. 17, p. 81.) 



In the centre of the carapace, and forming an equilateral triangle with the eyes, is an 

 elongate tubercle. The general surface appears somewhat rugose (see Woodcut, fig. 17). 



1 Thoracic plate, not epistome. — H. W. 



2 The head in Pterygotus anglicus, Ag., is now known to be rounded in front, not trapezoidal, as was 

 believed to be tbe case in 1859, when tbe above description was penned (see ante, Part I, p. 36, Pis. I and 

 II).— H. \V. 



3 This appears to be an error. In Pt. anglicus the penultimate segment is only keeled upon the dorsal 

 surface (see ante, Part I, p. 42, fig. 6, PI. Ill, fig. 2, and PI. VIII).— H. W. 



4 The Scotch species is proved to have a well-marked keel to the telson (see Part I, p. 42, PI. II, fig. 1, 

 and PL VI). In PI. II, fig. 1, the young and perfect Pterygotus there drawn has a high ridge to the telson, 

 which is clearly seen folded down towards the right side of the tail-plate. In PI. VI it is erect and thickened 

 into a strong keel by reason of the greater age of the individual — H. W. 



