228 THE CANADIAN NATURALIST. [Vol. vi. 



been dealt with. In the meantime the authenticity of the con- 

 clusions arrived at by Mr. Billings having been called in question 

 by Drs. D ma, Verrill, and Smith (see the American Journ, of 

 Science for May last, p. 320 ; Annals and Mag. Nat. Hist, for 

 May, p. 366). I have carefully considered their objections, and 

 have replied to the same in the Geological Magazine for July 

 last, p. 289, PI. VIII.; and I may be permitted here to briefly 

 state the arguments pro and con, seeing they are of the greatest 

 importance in settling the systematic position of the Trilobita 

 amonir the Crustacea, 



Until the discovery of the remains of ambulatory appendages 

 by Mr. Billings in an Asnplius from the Trenton Limestone (in 

 1870), the only appendage heretofore detected associated with any 

 Trilobite was the hypostome or lip-plate. 



From its close agreement with the lip-plate in the recent Apus, 

 nearly all naturalists who have paid attention to the Trilobita in 

 the past thirty years have concluded that they possessed only soft 

 membranaceous gill-feet, similar to those of Branchipus, Apns, 

 and other Phyllopods. 



The type-number of segments in Crustacea is 20 or 21. In 

 all the higher forms, as in the Decapoda, Stomapoda, Isopoda, 

 etc., several of these segments are coalesced either in the head, 

 thorax, or abdomen, so that we never meet with a Crustacean 

 having 21 distinctly -marked segments until we arrive at the 

 Branchiopoda and Phyllopoda, many of which have their full 

 number of separate segments. 



In the Trilobita, a very variable number of body-rings is met 

 with, from 6 even to 26 (in Harpes iinguJa, Sternb.), so that on 

 that account alone the Trilobita must be considered as a much 

 lower type than the Isopoda, in which the body-segments are 

 usually seven in number. There seems, however, no good reason 

 against the conclusion that the Trilobita were an earlier and more 

 generalized type of Crustacea from which the latter and more 

 specialized Isopoda have arisen. 



The large compound sessile eyes, and the hard, shelly, many- 

 seo-mented body, with its compound caudal and head shield, differ 

 from any known Phyllopod, but offer many points of analogy with 

 the modern Isopodn, and one would be led to presuppose the Tri- 

 lobites possessed of organs of locomotion of a stronger texture 

 than mere branchial frills. 



The objection raised by Drs. Dana and Verrill to the special 



