SUB-CLASS PECTINIFERA Differing from the Merostotana in having the first pair of ap pendages of the Mesosoma (abdomen) represented by a very small genital operculum, without a rod-like ovipositor or penis, and the second pair converted into the pectinei or combs; in the last of all external trace of the remaining f our pairs in the adult; in the small size of the basal segment of the last pair of legs and its loss of maxillary function, the breaking up of the lateral eyes into separate ocelli and the presence of poison glands in the postanal spine.
Order Scorpiones. The first pair of appendages (mandibles) small, three jointed and chelate; second pair (palpi) long, massive and chelate, the remaining four pairs (legs) locomotor, with the basal segments of at least the two first acting as jaws.
Sub-order Apoxypoda. Legs short, composed of stout sub-equal segments, the last or tarsus pointed and armed at most with a single claw, the basal segments of the last two pairs meeting in front of the sternal plate of the cephalothorax and the pectines with a short inner branch (endopodite). Family Palaeophoridae (Palaeophorus. ) This sub-order contains the Silurian scorpion which was un doubtedly marine. It has no trace of spiracles and probably breathed by means of branchial lamellae as in the Eurypterida.
Sub-order Dionychopoda. Legs long, with unequal segments, the last truncated and armed with two claws; the last two pairs of legs with their basal joints (coxae) abutting against the sternal plate; the pectines without endopodite.
To this sub-order belong all the recent scorpions which are re ferred to four families, Buthidae, Pandinidae, Vaegovidae and Bothorinidae. A number of genera from the Coal Measures mostly resembling recent forms in essential characters are also known. One of them (Eobutlius), however, has no trace of spiracles, the respiratory lamellae being probably concealed beneath the ven tral plates of the abdomen as in Palaeophorus. It may have been a water-breather, living in the Carboniferous marshes.