522 Reports and Proceedings. 



15. Cyclus torosus, H. "Woodw. Carboniferous Limestone, Little Island, Cork. 



16. Wrightii, H. Woodw. Carboniferous Limestone, Little Island, Cork. 



1 7. Earknessi, H. "Woodw. Carboniferous Limestone, Little Island, Cork. 



*18. radialis, Phillips. Carboniferous Limestone, Settle, Yorkshire, Vise, 



Belgium. 

 *19. Cyclus Rankini, H. "Woodw. Carboniferous Limestone, Carluke, Lanarkshire. 



[*20. ^^ Brongniartianus," liQ'Kom.nck. Carboniferous Limestone, Yorkshire, 



Belgium.] 



21. Cyclus Jonesianus, H. "Woodw. Carboniferous Limestone, Little Island, 

 Cork. (These latter figured and described in the Geol. Mag., 1870, Vol. VIL, PI. 

 XXIIL, Figs. 1-9.) 



[Those marked with an asterisk have been already figured, but have been redrawn 

 and redescribed in order to add to or to correct previous descriptions. Thus, for 

 example, " Cyclus Brongniartianus" proves upon careful examination to be only the 

 hypostome of a Trilobite belonging to the genus Fhillipsia. Dithyrocaris tenui- 

 striaius is identical with Avicula paradoxides of De Koninck, etc.] 



Since noticing the occurrence of an Isopod [Palcega Carteri), from the Kentish, 

 Cambridge, and Bedford Chalk, Dr. Ferd. Eoemer, of Breslau, has forwarded me the 

 cast of a specimen of the same Crustacean from the Chalk of Upper Silesia. This, 

 together with the example from the Miocene of Turin, gives a very wide geographical 

 as well as chronological range to this genus. 



A still more remarkable extension of the Isopoda in time is caused by the discovery 

 of the form which I have named Prcearcturus in the Devonian of Herefordshire, 

 apparently the remains of a gigantic Isopod resembling the modern Arcturm 

 Baffinsii. 



1 have also described from the Lower Ludlow a form which I have referred with 

 some doubt to the Amphipoda, under the generic name of Necrogammarus. 



Representatives both of the Isopoda and Amphipoda will doubtless be found in 

 numbers in our Palseozoic rocks, seeing that Macrouran Decapods are found as far 

 back as the Coal-measures, ^ and Brachyurous forms in the Oolites.* 



Indeed the suggestion made by Mr. Billings as to the Trilobita being furnished 

 with legs (see Quart. Journ. Geol. Soc, vol. xxvi., pi. 21, fig. 1), if established upon 

 further evidence, so as to be applied to the whole class, would carry the Isopodous 

 type back in time to our earliest Cambrian rocks. 



I propose to carry out an investigation of this group for the purpose of confirming 

 Mr. Billings's and my own observations, by the examination of a longer series of 

 specimens than have hitherto been dealt with. In the mean time the authenticity of 

 the conclusions arrived at by Mr. Billings having been called in question by Drs. 

 Dana, 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 among the Crustacea. 



Until the discovery of the remains of ambulatory appendages by Mr. Billings in 

 an AsapJius from the Trenton Limestone (in 1870), the only appendage heretofore 

 detected associated with any Trilobite was the hypostome or lip-plate. 



From its close agreemeut with the lip-plate in the recent Apus, nearly all natura- 

 lists who have paid attention to the Trilobita in the past thirty years have con- 

 cluded that thay possessed only soft membranaceous giU-feet, similar to those of 

 Branchipus, Apus, 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 ungula, 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 



1 Anthrapalcemon Orossartii, Salter, Coal-measures, Glasgow. 

 * Palceiiiachus longipes, H. Woodw., Forest Marble, Wilts. 



