EXTERNAL FEATURES 285 



condition; and since these specimens are found in a dolomitic 

 rock which is soluble in acid, it has been possible to separate 

 the fossil completely from the rock in which it is embedded, 

 with the result that the structure can be studied more easily 

 and more thoroughly than in the case of specimens from other 

 localities. Consequently Eurypterus fischeri ' may, with ad- 

 vantage, be taken as a type of the Eurypterida. 



The general form of the body (Fig. 161) is somewhat like 

 that of a Scorpion, but is relatively broader and shorter. On the 

 surface of many parts of the exoskeleton numerous scale-like 

 markings are found (Figs. 162, 163).^ The prosoma or cephalo- 

 thorax consists of six fused segments covered by a quadrate 

 carapace with its front angles rounded. This bears on its dorsal 

 surface two pairs of eyes — large kidney-shaped lateral eyes and 

 median ocelli (Fig. 161, h, a). The margin of the dorsal part 

 of the carapace is bent underneath to form a rim which joins the 

 ventral part of the carapace. 



On the ventral surface of the prosoma (Fig. 162) six pairs of 

 appendages are seen, of which only the first pair (the chelicerae) 

 are in front of the mouth. The chelicerae are small, and each 

 consists of a basal joint and a chela, the latter being found parallel 

 to the axis of the body ; they closely resemble the chelicerae of 

 Limulus. The remaining five pairs of appendages are found at the 

 sides of the elongate mouth, and in all these the gnathobases 

 of the coxae are provided with teeth at their inner margins and 

 were able to function in mastication, whilst the distal part of 

 each appendage served as an organ of locomotion. The posterior 

 part of each coxa is plate-like and is covered (except in the case 

 of the sixth appendage) by the coxa of the next appendage 



' A detailed account of Eurypterus fischeri has been given by G. Holm, Mem. 

 Acad. Im'pir. Sci. St. Pitersiourfj (8), viii. 2, 1898. See also F. Schmidt, Hid. (7), 

 xxxi. 5, 1883. Descriptions of American forms of Eioryjjterus are given by Hall, 

 "Nat. Hist. New York," Palacont. iii., 1859, p. 395 ; iUd. vii., 1888, p. 156 ; and 

 Second Oeol. Survey Pennsylvania, "Report of Progress," ppp., 1884 ; Whiteaves, 

 Geol. and Nat. Hist. Surv. Canada, "Palaeozoic Foss," iii., 1884, p. 42. 



^ It was this ornamentation found on fragments of Pterygotus anglicns which 

 led the Scotch quarrynien to apply the name "Seraphim" to that Eiirypterid. 

 On this subject Hugh Miller writes: "The workmen in the quarries in which 

 they occur, finding form without body, and struck by the resemblance which the 

 delicately waved scales bear to the sculptured markings on the wings of cherubs — 

 of all subjects of the chisel the most common — fancifully termed them ' Seraphim ' " 

 (The Old Red Sandstone, ed. 6, 1855, p. 180). 



