426 MR. H. M. BEENAED ON THE [Aug. 1 894, 



to Apus lies in the fact that in this form we have the specialization of 

 the mouth-parts which remained typical of the later Crustacea. In 

 Apus the second antennae degenerated, that is, as compared with the 

 anterior pair, their ventral masticatory portions almost, if not 

 entirely, disappearing. In the third pair of limbs it is the dorsal 

 portion which entirely disappears, while the ventral develops into a 

 large fleshy jaw. In the last two limbs the dorsal portions persist 

 in a rudimentary condition, while the ventral are masticatory ridges, 

 second in importance only to the ' mandibles.' On the trunk the 

 masticatory portion of the limbs progressively gives up its function, 

 while the dorsal portions develop primarily as organs of locomotion. 

 There is no reason to believe that any trilobites possessed this 

 formula for the cephalic appendages. Certainly in the older trilo- 

 bites, in which we find the head-region either incomplete as to the 

 number of the segments, or with the typical number of segments but 

 not very closely fused together, it was not likely that the limbs of 

 these segments were specialized like those of Apus and the higher 

 Crustacea, in which the head-segments are fused beyond all further 

 recognition as such. Judging, indeed, from those merostomata 

 whose cephalic limbs we know anything about, there is reason to 

 believe that the trilobites tried almost every possible masticatory 

 formula. 



As to the limbs of the trunk, Burmeister assumed that they were 

 membranous ' lobes ' like those of Apus and Branchipus. Becent 

 discoveries, however, show that the ambulatory portion of the leg 

 was filiform ; yet Burmeister was not far from the truth. The limb 

 of the trilobite, according to Walcott's sections, was a biramose 

 appendage, with a gill, a cirrus (exopodite), and a locomotory ' endo- 

 podite,' and, what is of equal, if not of greater importance, a flat, 

 membranous, basal portion. 



Commencing with the distal portion of the leg, Walcott's claim 

 that it was biramose has now been fully confirmed by the discovery 

 of specimens of Triarthrus Beckii 



showing appendages. 1 In these Fig. 12. — Limb of Triarthrus 

 beautiful specimens we have the Beckii. (After Beecher.) 



distal portions of the limbs shown 

 us closely resembling those of 

 Apus, only in Apus the two 

 branches are flat and mem- 

 branous for swimming, while in 

 Triarthrus they are apparently 

 longer and narrower and second- 

 arily jointed, for crawling. As 

 all who have examined Apus 

 know, the two branches are arranged side by side exactly as we find 

 in Triarthrus (fig. 12), the exopodite being behind the endopodite. 



1 See Walcott's valuable paper quoted above, and also the more recent paper by 

 Matthew, and further Dr. 0. E. Beecher, ' On the Thoracic Legs of Triarthrus,' 

 Auier. Journ. Sci. ser. 3, vol. xlvi. (1893) p. 467. 



