Vol. 50.] STSTEMATIC POSITION OF THE TRILOBITES. 



413 



times we now know for certain, not only from the existence of rich 

 remains of phyllopods with shields closely resembling that of Apus, 1 

 but further from the remarkable Cambrian Protocaris Marshi- 

 (fig. 1), which apparently possessed the same peculiar character of 

 the posterior segmentation as Apus, and which I should like to 

 call Apus Marshi. 



Fig. 1. — Protocaris Marshi, 

 Walcott. 



Again, the extinct Echinocaris 

 takes its name from a feature 

 which it possessed in common 

 with Apus. The posterior cylin- 

 drical (and apparently limbless) 

 segments are provided with a 

 ring of spines slightly anterior 

 to the posterior edge of the seg- 

 ment. Serrated posterior edges 

 of these segments occur very 

 generally in the copepoda, and a 

 variation of the arrangement in 

 Echinocaris occurs in some stoma- 

 topoda, and perhaps on the dorsal 

 sides of other Crustacea (not 

 phyllopods). It is, however, very 

 marked in the phyllopods Apus 

 and Estheria, in the former of which it repeats almost exactly the 

 arrangement in Echinocaris, there being a complete ring of sharp 

 spines round each of the posterior segments, slightly in front 

 of its posterior edge. In both Echinocaris and Apus, further, this 

 special ring of spines is not developed on the anal segment. 

 Moreover, the shell of Echinocaris has lateral markings which in- 

 voluntarily suggest the markings caused by the shell-gland on the 

 carapace of Apus. In addition to two caudal cirri, Echinocaris had 

 the median prolongation of the anal segment which is character- 

 istic of so many of the Apodidae (Lepidurus). 3 



II. The formation of the head by the gradual incorporation of 

 trunk-segments is now very clearly shown in Walcott's detailed 

 description of the Cambrian trilobites of North America. The 

 composition of the head out of five somites is, as is well known, 

 a crustacean characteristic, although no crustacean now shows this 



1 See ' Mouograph of the British Palaeozoic Phyllopoda,' pt. i. T. R. Jones 

 and H. Woodward, Palasont. Soc. 1888. See also the paper by Clarke(' American 

 Naturalist,' 1893, p. 793) on the carapace of Rhinocaris. It seems to me that 

 the remarkable double suture which he describes for this interesting Devonian 

 crustacean points back to the univalve condition of the original carapace. It 

 is easy to deduce both forms of the carapace, that with a single median, and 

 that with a double suture, from an Apus-\ike shield ; whereas it would be 

 difficult to arrange these carapaces in any other order of development. 



2 Walcott, ' On the Cambrian Faunas of North America,' Bull. U.S. Greol. 

 Surv. No. 10, vol. ii. 1884-1885. 



3 See James Hall's figures, ' Natural History of New York,' pis. xxix.-xxx. 

 vol. Til. (1888). 



U. J. Ki. H. No. 199. 2 f 



