784 



ARTHKOPODA 



PHYLUM VII 



increase in len-th from the first backward, the last pair enormously elongated, 



heincr nine-jointed, extending almost to the end of the telson, and termmating 



^ •' in a Sharp claw. The first two 



pairs of legs frequently bear 

 paired leaf-life spines. Telson 

 long and slender. Silurian and 

 Devonian ; Scotland, Russia, New 

 York, and Pennsylvania. 



Subgenus : Ctenopterus Clarke 

 and Ruedemann. First three 

 pairs of legs with numerous paired 

 spines to each segment. 



Tarsopterus Clarke and Ruede- 

 mann. Silurian ; New York. 



Drepanopterus Laurie. Body 

 as in Stylonurus, but the pos- 

 terior legs much. less elongated. 

 Silurian ; Scotland and Indiana. 



Family 2. Pterygotidae 

 Lankester. 



Body elongate, narrow. Pro- 

 soma semielliptical to subquadrate in 

 outline; telson spiniform in Hugh- 

 spatulate and hilohed 

 te in all other genera. Com- 

 eyes facetted, marginal ; 

 epistoma present ; chelicerae some- 

 times greatly extended. Last pair 

 of legs always adapted to swimming. 

 Female genital appendage simple. 



P ier ygotus Agassiz (Figs. 1511-1512). Eurypterids sometimes of gigantic 

 size, attaining a length of over 2 m. Prosoma semi-ovate, with anterior 

 marginal facetted eyes and median ocelli. The metastoma is a heart-shaped 

 plate attached along the middle line to the ventral wall of the prosoma, 

 between the bases of the last pair of legs, and extending outwards and 

 forwards so as to enclose the jaws in a kind of Chamber. Epistoma a thin 

 plate, occupying the same position as the hypostoma in trilobites, and having 

 the preoral appendages attached close to its posterior border. The latter are 

 large pincers, probably prehensile in function; by Schmidt and Woodward 

 they are represented as many-jointed, but it is now known that they consist 

 of but three long joints. Behind the mouth are four pairs of slender Walking 

 legs, followed by the large " swimming feet," which are similar to those in 

 Eurypterus, except that they are less broadly expanded at the tips. Telson 

 an oval plate, terminating in a slight projection. The species first referred 

 to this genus (P. proUematicus Ag.) is imperfectly understood, and P. anglims 

 Ag., which is well known, is generally accepted as the typical form of the 

 genus. Ordovician ; New York. Silurian ; Wales, Scotland, Sweden, Oesel, 

 Russia, New York, and Australia. Old Red Sandstone ; Scotland. 



Fig. 1510. 



Stylonurus excelsior Hall. Chemung-Catskill Group (Upper 

 Devonian) ; Pennsylvania. Restoration of dorsal aspect. 

 1/20 (after Clarke and Ruedeniann). 



