BRITISH FOSSILS. 



MONOGRAPH THE FIRST. 



PART II. DESCRIPTION of the SPECIES of PTERYGOTUS. By J. W. 

 SALTER, F.G.S., A.L.S., Geological Survey of Great Britain. 



PTERTGOTUS. GEN. Ag. 



The generic characters have already been detailed in the intro- 

 ductory portion of this memoir. It is only necessary here to give 

 them in brief, and point out the two very distinct sections or sub- 

 genera into which the group may be divided. These sections are 

 indicated by the form of the head, the position and outline of the 

 eyes, the shape of the labrurn and palpi, and probably, too, by that 

 of the terminal abdominal segments. 



Genus PTERYGOTUS. AGASSIZ, 1844. (Class Crustacea, Order Eurypterida.) Cara- 

 pace small, semi-oval or subquadrate, with lateral eyes, followed by 12 body segments 

 unfurnished with appendages, the twelfth being a large telson, pointed or acuminate, or 

 blunt and bilobed : epistoma large, deeply trilobed ; antennae of few (4) joints, strongly 

 chelate ; mandibles (and maxillaj ?) with palpi of 6 or more joints ; metastoma ovate, large, 

 notched anteriorly ; a large pair of ectognaths, their terminal joints expanded for swim- 

 ming, and the great foliaceous basal joint furnished with an inner serrate lobe or process. 



Section 1. Carapace semi-oval, produced in front beyond the elongated lateral eyes ; 

 labrum with its middle lobe ovate; palpi linear, simple; bases of swimming feet 

 foliaceous and overlapping each other; the succeeding joints attached posteriorly. 

 [Terminal joint or telson truncate or bilobed ; the penultimate not expanded.] 3 species. 

 P. bilobus, &c. 



Section 2. Pterygotus proper. Carapace quadrate or subquadrate, seldom semi-oval 

 or produced in front ; eyes large, round, placed far forwards ; labrum transverse, its 

 middle lobe sagittate at base ; palpi with tumid joints, fringed, or branched ; bases of 

 swimming feet not overlapping each other ; the succeeding joints attached more forward 

 than iu the last section. [Telson ovate, apiculate, or greatly produced, the penultimate 

 joint expanded.] 12 species. P. anglicus, P. gigas, &c. 



It will be seen that these sub-genera contrast in nearly all points. 

 The production, however, of the front margin into a semicircular 

 form in one of the true Pterygoti, P. gigas, and the possibly 

 emarginate apex to the long ovate telson of that species, show that 

 these characters are not without variation. Yet even in this case 

 the large rounded eyes and wide expanded penultimate body joint 



355815 



