XI EXTERNAL FEATURES 28 
as 
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- 
rantage, 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, 6, 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 Kurypterus fischeri has been given by G. Holm, Mém. 
Acad. Impér. Sci. St. Pétersbourg (8), vill. 2, 1898. See also F. Schmidt, bid. (7), 
xxxi. 5, 1883. Descriptions of American forms of Hurypterus are given by Hall, 
““Nat. Hist. New York,” Palacont. iii., 1859, p. 395 ; ibid. vii., 1888, p. 156; and 
Second Geol. Survey Pennsylvania, ‘* Report of Progress,” prr., 1884 ; Whiteaves, 
Geol. and Nat. Hist. Surv. Canada, ‘* Palaeozoic Foss,” iii., 1884, p. 42. 
2 It was this ornamentation found on fragments of Pterygotus anglicus which 
led the Scotch quarrymen to apply the name ‘‘Seraphim” to that Eurypterid. 
On this subject Hugh Miller writes: ‘‘ The workmen in the quarries in which 
tliey 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). 
