PLESiOSIRO. 43 



I liave established this new Order, named in alhision to the tactile nature of 

 the legs of the first pair, for a genus of Carboniferous Arachnida which cannot be 

 fitted into any other Order of extinct or existing forms. In some respects it 

 coincides with the definition of the Opiliones, notably in the total absence of 

 constriction between the prosoma and opisthosoma, and in the number and nature 

 of the tergal plates of the opisthosoma, and could hardly, indeed, be excluded 

 from that Order by the structure of the appendages as seen from the dorsal side, 

 although differing from all its knoAvn members in the TJieh/plioiuis-Yike modification 

 for tactile purposes of the legs of the first pair. The structure of the anal 

 segment is also like that of some Opiliones, c. <j. Cyphoplithahnus, but it is also 

 like that of the Anthracomarti. The arrangement, however, of the sternal plates 

 of the opisthosoma and the position of the basal segments (coxas) of the prosoma 

 with regard to the sternal area of that region, so far as I can decipher them, are 

 very different from what may be seen in the Opiliones. They may, indeed, be 

 described as more in conformity Avith the normal and primitive than is exhibited 

 in that Order, which in these particulars is the most specialised of all Arachnida, 

 with the exception of the Acari. 



Famihj Plesiosironid.e, nov. 



Genm PLESIOSIRO, nov. 



Generic Characters. — Carapace high, narrowed in front, its anterior border 

 produced into a median beak-like spiniform process ; marked on each side by a 

 longitudinal crest extending from a point close to its antero-lateral angle to its 

 posterior border; the area external to these crests sloped abruptly down to the 

 lateral edge ; the area between the crests flatfish and marked with a pair of 

 lightly-curved crests, which arise at the base of the spiniform process, diverge, 

 then converge, and meet a little in front of the posterior border ; posterior border 

 straight. Opisthosoma narrowed posteriorly, the terga much Avider than long, 

 with straight posterior border, the last only being convex posteriorly. The 

 sternal plate of the last, lying just in front of the transversely oval or subcircular 

 anal oi)erculum, strongly angled, the preceding two sterna progressively less 

 strongly angled. Opisthosoma bordered laterally by an unsegmented ridge 

 representing probably the pleural integument of this region. 



Palpi with second segment (trochanter) short ; third, fourth, and fifth segments 

 longer and subequal in length ; the sixth short and tapering. Legs longish, the 

 femora and tibi^ the longest segments ; tibia of first compressed ; basal segment of 

 terminal tactile portion of the limb much longer than the rest individually. 



Type Species. — Plesiosiro viadeletji, sp. nov. 



