54 REPORT 1871. 



Thus, for example, " Cydus Brongniartianus " proves upon careful examina- 

 tion to be only the hypostome of a Trilobite belonging to the genus Phillijjsia. 

 Ditliyrocaris tenuistratus is identical with Avicula pco-adoxides of De Koninck, 



&c.]' 



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 weU 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 Prceardurus in the Devonian of 

 Herefordshire, apparently the remains of a gigantic Isopod resembling the 

 modern Arcturus Bcrffinsii. 



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

 with some doubts to the Amphipoda, under the generic name of Necrogam- 

 marus. 



Bepresentatives both of the Isopoda and Amphipoda vrill doubtless be 

 found in numbers in our Palaeozoic rocks, seeing that Macruran Decapods 

 are found as far back as the Coal-measures*, and Brachyurous forms in the 



Oolites f. 



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

 nished with legs (see Quart. Journ. Geol. Soc. vol. xxvi. pi. 31. 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 arnved at by Mr. Billings having 

 been called in question by Drs. Dana, Verrill, and Smith (see the American 

 Joum. of Science for May last, p. 320 ; Annals & 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. 8 ; and I may 

 be permitted here to briefly state the arguments p>-o and con, seeing they are 

 of the greatest importance in settling the systematic position of the Trilo- 

 bita among the Crustacea. 



Until the discovery of the remains of ambulatory appendages by Mr. Bil- 

 lings in an Asaphus from the Trenton Limestone (in 1870), the only appen- 

 dage heretofore detected associated with any Trilobite was the hypostome or 

 lip-plate. 



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

 from the fact of the number of body-rings exceeding that attained in any 

 other group save in the Entomostraca, nearly all naturahsts who have paid 

 attention to the Tiilobita in the past thirty years have concluded that they 

 possessed only soft membranaceoiis giU-feet, similar to those of BrancMpus, 

 Apiis, and other Phyllopods. 



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

 body, with its compound caudal and head-shield, diifer from any known 

 PhyUopod, but offer many points of analogy with the modem Isopods J ; and 



* Anfkrapalamon Grossartii, Salter, Coal-measures, Glasgow, 

 t Palminachus longipes, H. Woodw., Forest Marble, Wilts. 



J It should always, howerer, be borne in mind that as the Trilobita offer, as a group, no 

 fixed number of body-rings and frequently possess more than twenty-one segments, they 



I 



