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 



FIG. 1424. 



Pterygotus anglims, Agassiz. Old Red Sand- 

 stone ; Forfarshire, Scotland. Restoration of 

 ventral surface, but second pair of legs not 

 represented. i/ 5 (after Woodward). 



FIG. 1425. 



Pterygotus Bu/aloensis, 'Grote. Waterlime (Silurian) ; 

 Buffalo, New York. The toothed antennal chela. 1/2- 



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 Eucrustacea 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. TRANS.] 



Sub-Class 2. ARACHNIDA. Spiders, Scorpions, Mites, etc. 



A cerata 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. Gephalothorax usually without signs of segments 

 dorsally. Abdomen sometimes clearly segmented, sometimes with the segments obsolete, and 

 apodal. 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. 



