678 ARTHROPODA SUB-KINGDOM VII 
“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. pro- 
blematicus, Ag.) is but imperfectly understood, 
and P. anglicus, Ag., which is well known, is 
generally accepted as the typical form of the 
genus. Uppermost Silurian; Wales, Scotland, 
Oesel, New York. Old Red Sandstone ; Scotland. 
Erettopterus, Huxley and Salter. As in 
Pterygotus, but with a bilobed telson. Silurian ; 
Lanarkshire. 
Glyptoscorpius, Peach. Body attaining a 
length of 30 cm. Surface covered with highly 
developed scale-markings. Limbs ending in a 


s QS = 

AAS 
Fic. 1424. iit 2 eee 
Pterygotus anglicus, Agassiz. Old Red Sand- Fic. 1425 
stone; Forfarshire, Scotland. Restoration of hae 
ventral surface, but second pair of legs not Pterygotus Buffaloensis, ‘Grote. Waterlime (Silurian); 
represented. 1/5; (after Woodward). Buffalo, New York. The toothed antennal chela. 1/5. 
double claw, as in scorpions, and the animal further provided with a pair of comb-like 
structures closely resembling the pectines of scorpions. Coal Measures; Scotland. 
[The chapter on the Merostomata was outlined in substantially its present form by Dr. 
John M. Clarke, of Albany, who furthermore contributed a number of new illustrations, both 
for this section and others on the Hwcrustacea which he has revised for this work. Some 
additional details were introduced under the generic diagnoses by the Editor, and the whole 
text finally subjected to the criticism of Dr. J. S. Kingsley and the Author.—TRays. ] 
Sub-Class 2. ARACHNIDA. Spiders, Scorpions, Mites, etc. 
Acerata with six pairs of limbs around the mouth, at least four of which are ambu- 
latory in function. Eyes simple, variable in number. Respiration by lung-books or by 
tracheae, these in either case developed from gill leaves arising on the posterior surface of 
one or more abdominal appendages. Cephalothorax usually without signs of segments 
dorsally. Abdomen sometimes clearly segmented, sometimes with the segments obsolete, and 
wpodal. Malpighian tubes, when present, ectodermal in origin. 
Six of the eight orders into which living Arachnids are divided occur in the fossil 
condition, and of these the scorpions, spiders, and Pedipalpi have existed ever since 
the Palaeozoic. By far the greatest number of species is preserved in amber of Lower 
Oligocene age from East Prussia. This fossil gum acts as a marvellous preservative, 
the most delicate parts, including the finest hairs and even spiders’ webs, remaining 
practically unaltered. 
The order Acari (Mites, Ticks) comprises forms in which the cephalothorax is 
anchylosed with the unsegmented abdomen. Fossil representatives occur in amber or 
fresh-water deposits of the Tertiary, and belong with few exceptions to recent genera. 
