798 CLASS V. ARACHNIDA. 



projecting backwards between the halves of the operculum,. 

 and covering paired styles which project in the corresponding 

 position from the second pair of appendages. Only a small 

 shield-like median structure is found between the halves of 

 the operculum in the supposed males. 



The six posterior abdominal segments are closed rings and 

 the anus opened on the last. The telson is spine-like in Eury- 

 pterus, a flattened plate in Pterygotus. 



In addition to the sexual differences above noted (in size, 

 and the structure of the organs on the first and second abdominal 

 segments), the supposed males have, in Eurypterus, a hook-like 

 (clasping ?) structure on the third pair of cephalothoracic appen- 

 dages, those which in the males of Limulus are also modified. 



Some of the Eurypterida attained a length of nearly five feet. 



Fam. 1. Eurypteridae. Eurypterus Dekay, some 20 spp. from Silurian 

 to Carboniferous ; Stylonurus Page, Silurian and Devonian ; Dolichopterus 

 Hall, with deeply cleft metastoma, Silurian N. Am. ; Slimonia Page, 

 Devonian of Scotland. Pterygotus Agassiz, with long powerful chelicerae, 

 Ordovician and Devonian of Europe and N. America. 



Sub-class 3. EUARACHNIDA 



Air-breathing Arachnida with fused head and thorax (prosoma), 

 with two pairs of jaws, four pairs of ambulatory legs, and apodal 

 abdomen (meso- and meta-soma). 



In this group the respiratory lamellae and the abdominal 

 limbs bearing them have apparently sunk into pits in the body 

 and given rise to the pulmonary sacs with their contained lung- 

 books. In some forms the lung-books are replaced by tracheae. 



Order 1. SCORPIONIDEA * 



Arachnids with pro-, meso- and meta-soma clearly distinct ; 

 the prosoma is covered by a single dorsal tergum which bears median 

 and lateral eyes ; the small chelicerae and the large pedipalps are 

 chelate ; the 3rd, 4th, 5th, and 6th jointed appendages are walking 



* Lankester, Quart. J. Micr. Sci., xxiv, 1884 ; Trans. Zool. Soc., London, 

 xi, 1883. Lankester and Bourne, Quart. J. Micr. Sci., xxiii, 1883. Brauer, 

 Zeitschr. wiss. Zool., lix, 1895. Lankester, Quart. J. Micr. Sci., xxiv, 

 1884. Laurie, Quart. J. Micr. Sci., xxxi, 1890 and xxxii, 1891. Pocock 

 An. Mag. Nat. Hist. (6), xii, 1893. Kraepelin, Scorpiones, Das Tierreich. 

 L. 8. 



