1XXX PROCEEDINGS OP THE GEOLOGICAL SOCIETY. [May 1895, 



side downwards, the gases in the viscera produced during decom- 

 position were sufficient to overturn the animal and allow it to be 

 buried by the accumulation of the fine sediments in the position in 

 which it is now found. 



The appendages of Triarthrus appear now to be very well made 

 out. The antennae, as seen in a number of specimens, were 

 simple multiarticulate flagella, which Walcott in a later paper (Geol. 

 Mag. 1894, p. 246, pi. viii. fig. 1 e) has shown extend backwards to 

 the lateral margin of the hypostome, so that they occupy exactly 

 the same position as do the first antennae in recent Apus. 1 



Two small appendages, like simple palpi, with broad basal joints, 

 which may represent the maxilla, are seen in one of "Walcott's 

 specimens, and there were probably four pairs of similar cephalic 

 appendages, besides the simple flagellate antennae, more or less 

 modified to serve as mouth-organs. 



No essential differences have been observed in the series of limbs 

 attached to the (14 or 16 ?) free segments. 



Each of these segments bears a pair of biramous appendages 

 originating at the sides of the axis, as in other trilobites (e. g. 

 Asaphus, Galymene, Geraurus, etc.). 



The anterior legs are the longest, and the others gradually 

 become shorter towards the pygidium. 



Each limb consists of two nearly equal branches, the ' endopodite ' 

 and ' exopodite,' which may be correlated with the typical crus- 

 tacean primitive limb and are well displayed in the adult My sis ; in 

 the biramose natatory-feet of the zoea of the common shore-crab 

 (Carcinus) ; and retained in the appendages of the abdomen of the 

 adult lobster (Hornarus). 



The ' endopodite,' or walking-limb, is composed of seven articu- 

 lations, the first (coxopodite) or basal joint also giving origin to the 

 ' exopodite,' or swimming-appendage. 



The 'exopodite' has a long basal joint serrated on its lower 

 border, broader at the proximal end and narrower at the distal 

 extremity ; here it terminates in a many-jointed flabellate appen- 

 dage bordered with a thick fringe of setas, which extend also along 

 the serrated border of the basal joint. 



Practically, these biramose limbs are reproduced along the entire 

 series of free segments. The appendages belonging to the pygidium 

 closely resemble the branchigerous feet of Apus, and evidently may 

 be correlated with typical phyllopod limbs. 



1 See H. M. Bernard, Quart. Journ. Geol. Soc. vol. 1. (1894) p. 425. 



