•352 MR.„H. M. BERNARD ON THE SYSTEMATIC [Aug. 1 895, 



26. Supplementary Notes on the Systematic Position of the Tri- 

 iobites. By Henry M. Bernard, Esq., M.A. Cantab., E.L.S., 

 E.Z.S. (Communicated by Dr. Henry Woodward, F.R.S., 

 P.G.S. Read April 24th, 1895.) 



Since I had the honour of reading a paper on this subject before 

 the Geological Society (printed in this Journal, vol. 1. 1894, p. 411) 

 two other papers have appeared in America, giving further details 

 as to the appendages and structure of Triarthrus. 1 Both papers 

 are by Dr. C: E. Beecher. The earlier, describing the pygidial 

 limbs, appeared in time to be briefly mentioned in a footnote in my 

 own paper just referred to. The more recent has but now appeared, 

 and gives an account of the ventral structure of Triarthrus. These 

 two papers seem to me to contain matter of perhaps even greater 

 morphological interest than the discovery of the antennae. 



The Metastoma. — Taking the new evidence, not in the order of 

 its appearance, but in the order of the parts referred to, the meta- 

 stoma, ' now revealed for the first time,' is described and figured by 

 Dr. Beecher as a small, convex, arcuate plate just posterior to the 

 extremity of the hypostoma. A small arcuate plate projecting back- 

 wards, as shown in Dr. Beecher's figures, is the exact form that the 

 labium or lower border of the mouth must have assumed, when the 

 first segment of the hypothetical annelid ancestor bent round so 

 that the mouth opened posteriorly for the new method of feeding. 

 But such a metastoma, as I have already argued, would be a dis- 

 advantage : it would form a barrier in the way of food being pushed 

 into the mouth by the ' gnathobases.' In Apus we accordingly 

 find the metastoma modified into two lobes placed laterally and 

 squeezed in between the mandibles and maxillae. This arrange- 

 ment leaves the middle passage to the mouth open, while it pre- 

 vents food-particles from escaping laterally between the biting- 

 limbs. In Triarthrus we find the first stages of this modification ; 

 the primitive metastoma is not yet obliterated in the middle line, 

 but the lateral lobes are beginning to develop. 



Again, in the position of the labium, in front of, or at least in a 

 line with 2 the jaw-pieces of the second pair of appendages, Triar- 

 thrus has retained a very primitive feature. It is obvious that, on 

 the bending round of the first segment for the pushing of food into 

 the mouth, no pair of parapodia would at first be in exact position 



1 ' The Appendages of the Pygidium of Triarthrus] Am. Jour. Sci. ser. 3, 

 vol. xlvii. (1894) p. 298 ; and ' Further Observations on the Ventral Structure 

 of Triarthrus' American Geologist, vol. xv. (1895) p. 91. 



2 I see no reason for doubting that the relative position of these parts as 

 shown in Dr. Beecher's figures is the natural one. The metastoma and the 

 limbs inserted near it could not well move apart without tearing the cuticle. I 

 do not think that the chitin would be likely to stretch very much, as Dr. Beecher 

 suggests, except in the sense of smoothing out folds. But such a stretching 

 would, I think, reveal the true relations of the parts to one another. 



