ARANE^E. 12 1 
facial sutures, a trilobite-like thorax of a small but variable (7-12) number of segments, 
and a Limulus-like telson. Neither of them has yet been fully described or figured, but 
(Walcott 1912 A, p. 202) Habelia appears to have five pairs of cephalic appendages, the 
first two pairs of which are multisegmented antenna;. The thoracic appendages are likewise 
none too well known, but they appear to have been biramous. The endopodites are better 
preserved than the exopodites, but in at least one specimen of Molaria the exopodites are 
conspicuous. 
If these genera are properly described and figured, their appendages are typically crus- 
tacean, and fundamentally in agreement with those of Marrella. The relation to the Trilo- 
bita is evidently close, the principal differences being the absence of facial sutures and the 
presence of true antennae. I am therefore transferring the Aglaspidae from the Merosto- 
mata to a new subclass under the Crustacea. 
ARANEiE. 
The spiders have the head and thorax fused, the abdomen unsegmented except in the 
most primitive suborder, and so appear even less trilobite-like than the insects. The appen- 
dages likewise are highly specialized. The cephalothorax bears six pairs of appendages, 
the first of which are the preoral chelicerae, while behind the mouth are the pedipalpi and 
four pairs of ambulatory legs. The posterior pairs of walking legs belong to the thorax, 
but the anterior ones are to be homologized with the maxillae of Crustacea, so that the spiders 
are like the trilobites in having functional walking legs on the head. 
The chief likenesses are, however, seen in the very young. On the germ band there 
appear a pair of buds in front of the rudiments of the chelicerae which later unite to form 
the rostrum of the adult. At the time these buds appear, the chelicerae are postoral, but 
afterward move forward so that both rostrum and chelicerae are in front of the mouth. 
The rostrum is therefore the product of the union of the antennules, and the chelicerae are 
to be homologized with the antennae. There seems to be some doubt about the homology 
of the pedipalps with the mandibles, as at least one investigator claims to have found rudi- 
ments of a segment between the one bearing the chelicerae and that with the pedipalps. 
Jaworowski (Zool. Anzeiger, 1891, p. 173, fig. 4) has figured the pedipalp from the 
germ band of Trochosa singorimsis, and called attention to the fact that it consists of a cox- 
opodite and two segmented branches which may be interpreted as exopodite and endopodite. 
He designated as exopodite the longer branch which persists in the adult, but since the ambu- 
latory legs of Crustacea are endopodites, that would seem a more likely interpretation. As 
the figure is drawn, the so called endopodite would appear to spring from the proximal seg- 
ment of the "exopodite." If the two terms were interchanged, the homology with the limb 
of the trilobite or other crustacean would be quite perfect. 
In the young, the abdomen is segmented and the anterior segments develop limb-buds, 
the first pair of which become the lung books and the last two pairs the spinnerets of the 
adult. There seems to be some question about the number of segments. Montgomery 
(Jour. Morphology, vol. 20, 1909, p. 337). reviewing the literature, finds that from eight to 
twelve have been seen in front of the anal segment. The number seem to vary with the 
species studied. This of course suggests connection with the anomomeristic trilobites. 
The oldest true spiders are found in the Pennsylvanian, and several genera are now 
known. The head and thorax are fused completely, but the abdomen is distinctly seg- 
