AN ACCOUNT OF TWO NEW CRUSTACEA FROM THE 

 TRANSITION AND CARBONIFEROUS STRATA. 



Among all the relics of former worlds, there is, perhaps, none that 

 has more exercised the ingenuity of both naturalists and geologists, 

 in the determination of its original and perfect form, than the re- 

 mains of a crustaceous animal belonging to the transition strata 

 known bj'^ the various names of Entomolitkus paradoxus j Trilobite, 

 and Dudley Locust. The earliest appearance of this fossil is in 

 the Llandilo Flagstone, from which it extends upwards, through 

 most of the intervening strata, into the Fullers' Earth, accord- 

 ing to Mr. Parkinson, which is a formation above the Lias, and 

 beyond which it becomes perfectly extinct. It seems to have 

 been almost the sole representative of the extensive class of crus- 

 taceous animals through all this numerous series of rocks, whose 

 deposition (to judge from the thickness of some of the layers, 

 which are above five hundred feet) must have occupied immense 

 periods of time. Trilohites belong to the class of Etitomostraceous 

 Crustacea of Cuvier, and to the order of Pcecilopoda, so termed from 

 the varied form of their locomotive apparatus, some of which serve 

 for feet or swimming organs, and the others, being furnished with 

 fringed appendages, perform the office of gills. The other E?iio- 

 mostracea, with which the Trilobites are associated in certain of the 

 strata, belong to the genera of Cypris, Eurypterus, and Limulus, 

 which last possesses considerable analogy to certain genera of Trilo- 

 bites. To omit the hypotheses of other distinguished naturalists 

 and geologists, as Cuvier, Audouin, Goldfuss, and A. Brongniart, 

 &c.. Dr. Buckland ( Bridgewater Treatises, No. 6J has endeavour- 

 ed, with great success, to elucidate the structure of the Trilobites 

 by a reference to their affinities in the genera Serolis, Limulus, 

 and Branchipus ; and as one of the fossils I am about to describe 

 is analogous, in some respects, to these animals as well as to the 

 Trilobites, it will be necessary to give some account of them before 

 instituting a comparison between them and the specimen. For if 

 we assume the recent genus Serolis as the type of this class of 

 animals, we appear to pass gradually from its more perfect struc- 

 ture to the rudimentary form, (at least, apparently such,) of the Tri- 

 lobite, through the intermediate genera of Limulus and Branchipus. 



The Serolis (ibid., pi. 45, fig. 6), in size and general api)ear- 

 ance, resembles a small Crab. It is furnished with antenna? or horns, 

 and with two claws. Its eyes are placed upon its back, and, like 

 those of most of the animals I am describing, resemble the com- 



