722 HISTORICAL GEOLOGY. 



the contrary, have retained their transjiarency, neuration, and nse. Scudder 

 remarks, further, that a similar change took place after the Paleozoic, in the 

 Orthopteroids generally, though to a less extreme degree ; and it appears 

 therefore that the Carbonic era was the time of culmination not only for the 

 Cockroach family, but for the tribe as a whole. The change was a loss of 

 locomotive function by the anterior pair of wings, and an example therefore 

 of degeneration ; and it was attended, as Scudder states, by a great loss in 

 the size of the species, and especially of the wings ; the mean length of the 

 anterior wings in the Paleozoic species of Cockroaches being a little over an 

 inch (26 mm.), and 40 per cent less in later kinds. Among Hemipteroid 

 species, the Permian Eugereon Bockingi, of Germany, had the wings of both 

 pairs similarly diaphanous, while in the Phthcmocoris of the Permian of 

 Missouri, described by Scudder, the anterior pair were much thickened; the 

 result, probably, as in the Orthopteroids, of degeneration. It is probable 

 that Carboniferous Beetles had a like method of origin from Insects having 

 four diaphanous wings ; but the line of descent remains unknown. 



The Scorpions of the Upper Silurian are much like those of modern 

 time. The type is the lowest among the tribes of Arachnids, notwithstanding 

 their size. As in a Crustacean or Eurypterus, the body (Fig. 799) obviously 

 consists of a cephalothorax and a long abdomen. 



True Spiders have not yet been found in rocks earlier than the Carbon- 

 iferous ; and this is probably because Spiders are so little likely to be fossil- 

 ized ; for they are not only smaller animals than the Scorpion, but also they 

 are unlike them in not having a durable exterior. 



9. Derivation of Arachnids. — The line to the lower and earlier Arachnids 

 — that is, to the Scorpions — leads up, according to Van Beneden, Packard, 

 and others, from the early Pterygotus-like Limuloids. The early Scorpions, 

 as well as the modern kinds, have the same number of body segments as a 

 Eurypterus or Pterygotus : namely, seven thoracic and six abdominal (pre- 

 cisely the normal number in Crustaceans) ; the same cephalic relations of the 

 legs ; the same absence of abdominal appendages ; a like absence of thoracic 

 appendages from all the segments excepting the first two ; and similar func- 

 tions in the members pertaining to these two segments. Further, accord- 

 ing to B. Peach, these early Limuloids sometimes have, like the Scorpions, 

 pairs of "combs" or pectinated organs on the underside of some of the 

 thoracic segments. 



But in this change from an aquatic to a terrestrial species the upward 

 progress in structure was great. The four posterior pairs of feet in the 

 terrestrial Scorpion have no longer the low-grade feature of serving for jaws 

 as well as feet, but are simply feet ; they are the chief organs of locomotion, 

 and only those of the anterior pair are appendages to the mouth. The 

 antennae are shortened to pincers (falces) that also serve the mouth. The 

 four pairs of feet are thus cephalic organs, if comparison be made with 

 the Limuloids and Crustaceans ; though in Arachnology, they are called 



