ARACHNIDA (CONTINUED] DELOBRANCHIATA = MEROSTOMATA 



( CONTINUED) EURYPTERID A 



Order II. Eurypterida. 



THE ' Eurypterida or Gigantostraca are found only in the 

 Palaeozoic formations. Some species of Pterygotus, Slimonia, 

 and Stylonurus have a length of from five to six feet, and are 

 not only the largest Invertebrates which have been found fossil 

 but do not seem to be surpassed in size at the present day except 

 by some of the Dibrarichiate Cephalopods. All the Eurypterids 

 were aquatic, and, with the possible exception of forms found in 

 the Coal Measures, all were marine. The earliest examples 

 occur in the Cambrian deposits, and the latest in the Permian ; 

 but although the Eurypterids have thus a considerable geological 

 range, yet it is mainly in the Silurian and the Old Eed Sand- 

 stone that they are found, the principal genera represented in 

 those deposits being Eurypterus, Stylonurus, Slimonia, Pterygotus, 

 Hughmilleria, Doliclwpterus, and Eusarcus. From the Cambrian 

 rocks the only form recorded is Strdbops ; 1 in the Ordovician 

 the imperfectly known EchinognatJius' 2 and some indeterminable 

 fragments have alone been found. In the Carboniferous deposits 

 Eurypterus and Glyptoscorpius occur, and the former survived 

 into the Permian. 3 



1 Walcott lias described, under the generic name Beltina, imperfect specimens 

 from the Algonkian (pre-Cambrian) of Montana, which he thinks may be the 

 remains of Eurypterids (Bull. Geol. Soc. America, x., 1899, p. 238). 



2 Walcott, Amer. Jour. Sci. (3), xxiii., 1882, p. 213. 



3 Descriptions and figures of British Eurypterids are given in the following 

 works: Huxley and Salter, "Pterygotus," Mem. Geol. Survey, Brit. Org. Ee- 

 mains, i., 1859 ; H. Woodward, "Monograph of the Merostomata," Palaeont. Soc. 



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