TRILOBITES OF THE LOWER SILURIAN ROCKS. 



665 



at length. He certainly throws some new light upon the nature of these creatures, by exposing the 

 interior or under surface, particularly that of their heads, in which he points out several divisions, 

 and considers them to be thoracic plates and jaws. The central portion, or that which was formerly 

 described by Mr. C. Stokes from a North American specimen, Geol. Trans., vol. i. p. 208. PI. 27- 

 f. lb., he conceives to have been connected with the head by cartilage only, and to have served as 

 a thoracic plate to protect the stomach, the form of which varies in the different genera of trilobites 

 found in Russia 1 . 



It is remarkable that English collectors should not yet have met with specimens in which this 

 plate is discoverable, though it has been observed in Russian and American trilobites. I feel con- 

 fident, however, that even with the specimens which we possess, a good naturalist may develope 

 much of the organization of these crustaceans, by patient inquiry, and by clearing away with de- 

 licacy of hand the matrix of rock which so often obscures their under surface. The specimens 

 PL 7 bis, f. 3 b. and PI. 14. f. 3b., partially illustrate my meaning; and even while these pages 

 are going through the press, I see enough in my own cabinet to make me regret that I have not 

 had leisure to attend more assiduously to this part of the structure of Trilobites, by a study of 

 which we can alone hope to gain a complete acquaintance with the habits and structure of these 

 animals, as previously suggested by Audouin and Goldfuss. (Ann. des. Sciences Nat. vol. xv. p. 83.) 



On referring this subject to my friend Mr. W. MacLeay, whose knowledge of invertebrated ani- 

 mals is so profound, he assures me, that this plate on the underside of the head, above alluded to 

 (Stokes, Geol. Trans, vol. ii. PI. 2y. Lib.; Goldfuss, Ann. Scienc. Nat. torn. xv. PI. 2. f. 8. ; 

 Pander, Beitr. Tab. 4. ; Buckland, Bridgw. Treat. PI. 45. f. 12/.), must be considered the labrum or 

 upper lip. The trilobite is thus brought into close analogy with certain Entomostraca, such as the 

 ylpus cancriformis, Latr. The reader who compares the figure of the under side of that animal 

 (Savigny, Animaux sans Vertebres, 2 mem. PI. 7-) will observe a similar "labrum" and many other 

 striking analogies with our fossils, particularly in the lateral, inflected terminations of the shelly seg- 

 ments of the body (geologice ribs), a distinctly trilobed "pygidium " or caudal portion, and a pro- 

 longed tail ; while the feet being foliaceous and the abdomen merely covered by membrane, could 

 scarcely be expected (at least very rarely) to leave traces of their existence in a fossil state. 



On the other hand, if viewed on its upper side or back (as is the case in nearly all our specimens), 

 trilobites are more analogous to certain Isopoda, such as the Cymothoada} 2 , particularly in the 

 eyes and buckler ; and thus it is that our fossils appear, as before said, to constitute the link be- 

 tween two orders of existing crustaceans. 



The annexed observations on the place which trilobites hold in nature have been contributed by 

 my friend Mr. W. MacLeay in illustration of this work, while the preceding pages were passing 

 through the press. 



1 These genera, according to Pander, are Calymene, Asaphus, Illcenus, Amphion and Zethus, the two last- 

 mentioned being added by himself to those previously described. It would appear that another Russian author 

 (Stschegloff) described trilobites at as early a period as Dalman, and also subdivided the genus Asaphus, 

 Brongn. ; calling the Illcenus (Dalm.), Deucalion. 



2 Parkinson seems to have been the first to suspect an affinity between Trilobites and the genus Cymothoa. 

 See Organic Remains, Oct. Ed. p. 266. 



