282 GEOGRAPHICAL AtfD GEOLOGICAL DISTRIBUTION. 



The remains of insects in the older Paleozoic rocks are very 

 scanty, and in the main they occur in such an unsatisfactory state 

 of preservation as to have led to the most divergent views respect- 

 ing their true relationship. The most ancient of these forms, and 

 the only one that is thus far known from the Silurian rocks, is the 

 Palaeoblattina Douvillei, recently described by Brongniart from the 

 Middle Silurian sandstones of Calvados, France, and referred by 

 that naturalist to the orthopteroid group of the cockroaches. No 

 other member of the orthopterous division of insects is positively 

 known prior to the Carboniferous period, and it is not unlikely that 

 the fossil in question, which is represented by a single impression of 

 a wing, may on further investigation prove to belong to the group 

 of netted- veins (Neuroptera or Pseudo- Neuroptera), which alone 

 among the different orders has representatives in the Devonian 

 rocks. These last comprise fragments representing some five or 

 six species, belonging to possibly as many genera Palephemera, 

 Gerephemera, Lithentomum, Homothetus, Xenoneura, and Dis- 

 crytus. The first two are referred by Hagen to the modern type of 

 the Libellulae (Pseudo-Neuroptera), and the remaining three, not 

 counting the very doubtful and fragmentary Discrytus, to the like- 

 wise modern sialine tribe of the Neuroptera. By Mr. Scudder, on 

 the other hand, several of these earlier hexapods are considered to 

 represent synthetic types, and have accordingly been referred to 

 families specially created for their reception Palephemeridae, Ho- 

 mothetidae, Xenoneuridse ; indeed, the same authority, following 

 in the footsteps of Goldenberg and Brongniart, insists that these 

 forms, as well as all others, with possibly one exception, from the 

 Paleozoic deposits, in a given departure from modern type-struc- 

 tures, and in the possession of combination ordinal characters, con- 

 stitute a special group or order apart by itself, the Palaeodictyoptera, 

 whose diverging specialisation (effected at about the beginning of 

 the Mesozoic era) outlined the various higher groups or orders now 

 recognised by entomologists. 106 In conformity with this view the 

 Paleozoic insect was a synthetic hexapod, in which ordinal differ- 

 entiation had not yet asserted itself. The Carboniferous cock- 

 roaches (Palrcoblattarige), which constitute probably one-half of 

 the entire Paleozoic insect fauna, and of which some sixty or more 

 species are known, and the contemporaneous walking - sticks 

 Protophasma, the giant Titanophasma from Commentry, France, 



