2J:0 TEANSACTIONS OF THE [MaY 22 



cylindrical leg, and the other is thin, broad and leafy, with what 

 seems to be a comb-like structure similar to the gills of many 

 Crustacea. These branchiju, if so they be, depend from a nar- 

 row, thickened anterior edge or limb ; they seem to correspond 

 with two of the three cephalic (?) legs shown in the specimen, 

 and to overlie them. By analogy with Mr. Walcott's determina- 

 tions, they are probably attached to the basal joints of the leg. 



In Figs. 2, 3 and 6 are shown the ends of appendages, which 

 projected from under the carapace, and seem to belong to the 

 thoracic region. These are likewise of tvvo kinds, one of which 

 shows an oblique comb-structure or system of parallel lines 

 (see Figs. 2, 2'', 3 and 4) ; and the other is a strong tapering leg, 

 with three cylindrical or slightly flaring joints visible (see Figs. 

 2, 2*, 3, 5 and 6). The first may be a branchial appendage ; it 

 is flat and appears to broaden into a small paddle at the tip, 

 though this appearance may be deceptive ; it has a sharp ridge 

 and narrow furrow along the anterior edge, and behind that, the 

 series of lines or comb-like structure, which may be due to the 

 remains of hairs or gill-structure on the limb. 



Figure 4 shows a series of appendages of both kinds, but 

 very poorly preserved. In Fig. 7, the projecting appendages of 

 the tail-piece are shown, and it may be seen that apparently 

 several, and perhaps all of the pygidial limbs are anchylosed, so 

 as to make a rounded flap, which in shape, though not in struc- 

 ture, reminds one of the telson of a crayfish, and perhaps served 

 the same purpose — to propel the animal backwards through the 

 water. 



The conclusions as to the nature of these appendages are 

 only i^rovisional, and may be much changed by further discov- 

 eries. The}' are not nearly as well preserved as the antennfie, 

 nor in so considerable a number of specimens ; as besides those 

 figured, there are only two or three others which show them at 

 all, and those, only traces- The shai:)e and structure of the sup- 

 posed branchiae, in particular, are very hard to distinguish ; the 

 figures given, however, represent, as far as I can see, the actual 

 outline preserved. 



It will be seen that the structure of Triarthrus must have 

 differed not a little from that described by Mr. Walcott in Cahj- 

 mene and Ceraurus. The presence of antenna^, the broad, 

 leafy gills, and the anchylosed flap under the pygidium (provid- 

 ing that the two latter are correct interj^retations of the struc- 

 ture) are all important points of difference, and indicate that 

 the Trilobites were quite varied in structure, and probably 

 included a number of widely differing forms. If the classifica- 

 tion founded on the characters of the shield is not deceptive, 



