COMPAKATIVE MORPHOLOGY OF THE GALEODID^. 309 



of cliitin projecting from under the cephalic lobes. Laterally, these rods {>', PI. XXVII. 

 figs. 5, 0, 7) are continued downward to the skeletal framework of the ventral surface 

 posterior to the bend (PI. XXVIII. fig. 15, r). These rods, which are supports for the 

 so-called " head," vary slightly in their points of fusion with the chitinous plate or pair 

 of plates just mentioned, which project from under the cephalic lobes (cf. PI. XXVII. 

 figs. 1, 3, 4, 6). 



These projecting plates (marked 3 in the figures) are, in all the Galeodidie, separated 

 by a suture from tiie posterior border of the cephalic lobes themselves ; and in transverse 

 and longitudinal sections they are seen to run under the posterior blind ends of the lobes 

 (PI. XXVII. figs. 11, 12). In some genera, e.g., Bhax, the single piece is but a narrow 

 strip (figs. G, 7). In Galeodes the strip is wider and bent upward at an angle from the 

 cephalic lobes (fig. 5). It is sometimes wddest in the median line (fig. 3) ; at others, 

 at the sides, where it may form a pair of triangular plates (fig. 1). These are, it seems 

 to me, the remains of the tergum of the 3rd segment, the anterior portion of which, 

 together wath the terga of the 1st and 2nd segments, has been obliterated by the 

 cephalic lobes. 



The Cephalic Lobes. — These two plates, joined by a suture along the middle line, form 

 one of the most striking features of the Galeodidte. Their anterior edges, which vary in 

 outline in different genera, are generally sharp, and pass at once into the flexible 

 membrane round the bases of the chelicerse. In the genera Biton and Cleobis, the anterior 

 edge tends to bend over to form a slight " forehead " (fig. 2). From the anterior end of 

 the median suture, the ocular tubercle protrudes. Where there is a slight " forehead," the 

 eyes project forward from it (fig. 2). 



The median suture is sometimes, comparatively speaking, indistinct, the plate formed 

 by the two lobes being posteriorly rounded off {Rhax, fig. 6). On the other hand, the 

 suture may be very marked, even dividing the posterior margin of the double plate into 

 two lobes [Soljpuga, fig. 3). 



In all the Galeodidae there is a curious area [a in the figures) on each side of the cephalic 

 lobes, which is not easy to explain. Pocock sviggests (6o) that these are the remains 

 of the proximal joints of the cheliceraj. In this interpretation I was at first inclined to 

 agree, but now think that they have arisen secondarily from an infolding of the cuticle 

 to form the strong buttresses upon Avhich the chelicerse rotate (see further, infra, p. 321). 



Origin of the Cephalic Lobes. — In endeavouring to answer the question, " what are these 

 remarkable plates Avhich so disturb the segmentation at the anterior end of the body ? " I 

 have been led to refer them to the translocation of the chelicerse from a postoral to a 

 preoral and dorsal position. I would suggest the following explanation of the changes 

 which have taken place in the arrangement of the segments : — 



In order to carry the enormous chelicerae, lateral processes of the first segment have 

 been thrown backward on to the dorsal surface, where they meet in the middle line, 

 giving rise to the so-called " head," which Dufour attributed to these animals. A close 

 examination of this " head " shows that it consists of a pair of chitinous lobes carrying 

 the muscles of the chelicerae ; no other muscles run into them, and, moreover, the muscles 

 of the chelicerae are almost, if not entirely, confined to them. Fig. 12 (PI. XXVII.), 



