SUB-CLASS I MEROSTOMATA 677 
bent under so as to cover more than one-half of the ventral surface. Eyes large, 
approximate, sometimes surrounded by strong orbital ridges; ocelli on the slope of a 
median ridge. Preoral appendages known only by fragmentary chelicerae. Of the 
five pairs of postoral appendages, the first is chelate, the next two are short, and each 
joint bears a pair of lamellar processes. The last two pairs are enormously elongated, 
nine-jointed, extending almost to the end of the telson, and terminate in a sharp claw. 
Number of abdominal segments believed to be the same as in Ewrypterus ; each of the 
posterior segments sometimes bear a pair of detachable processes or epimera. Telson 

Fig. 1422. 
Slimonia acuminata, 
Salter. Devonian ; Lanark- 
shire, Scotland. Restora- 
tion of ventral surface, 
showing legs I.—VI. and 
Fic. 1423. 
abdominal segments VII.— Pterygotus Osiliensis, Schmidt. Silurian; Rootzikiill, Island of Oesel. Re- 
XIX. The first five of the storation of ventral surface. a, Epistoma; m, Metastoma; oc, Eyes; I.—VI., 
latter with branchial Appendages, with joints numbered serially in the powerful swimming pair; 
lamellae. 1/7 (after Laurie). I’.—V’., 7’, First six abdominal segments. 1/3 (after F. Schmidt). 
long and slender. Silurian and Devonian; Scotland. Upper Devonian; New York 
and Pennsylvania. 
Pterygotus, Agassiz (Figs. 1423-1425). Gigantic Eurypterids, attaining a length 
of over 2 m. Cephalothorax semiovate, with anterior marginal eyes and central 
ocelli. Comb-like organs are present beneath the cephalothorax as in Gilyptoscorpius. 
The metastoma is a heart-shaped plate attached along the middle line to the ventral 
wall of the cephalothorax, 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 semicircular plate, occupying the same position as 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 Laurie states them to consist of but three long joints. 
Behind the mouth are four pairs of slender walking legs, followed by the large 
