280 A REVIEW OF THE TRILOBITES. 



Little can be suggested with any certainty respecting the food of 

 the trilobite ; but by comparison with existing crustaceans, and from 

 the form of the labrum, it may be inferred that these creatures were 

 carnivorous Soft-bodied radiata, the coral polyp, decaying matter 

 drifted into sheltered bays — such may have formed, in part at least, 

 the sustenance of the trilobite. A further insight into this question 

 might be obtained, could we trace out the compensating agents in 

 Nature's economy, which served to replace the trilobites after these 

 had passed away. 



6. Geological, relations. The trilobites appear to have been called 

 into existence almost at the earliest dawn of animal life. They die 

 out at the base of the great carboniferous formation, and thus belong 

 entirely to the earlier and middle portions of the palaeozoic age. The 

 separate species offer, with few exceptions, admirable test-forms for 

 the various subdivisions of the Silurian and Devonian groups Even 

 the genera are in many instances restricted to comparatively narrow 

 limits in their upward range. Thus, the genus Trinucleus is unknown 

 above the deposits which mark the limit of the lower Silurians. 

 AsapJius, Illcenus, Pa.radoxiJ.es. follow the same law ; but other gen- 

 eric forms, Calymene, Phacops, &c, pass upwards, although as a rule 

 with different species, into the higher Silurian and Devonian periods. 

 Phillipsia, very rare in earlier groups, becomes, in the Lower Carbon- 

 iferous, almost the only remaining type of the class. But these cha- 

 racteristics will be found in full under our enumeration of the more 

 common species belonging to each genus. At present, let us briefly 

 glance at the geological relations of the Crustacea generally. 



The decapods comprise three well-marked groups : — the braehyura. 

 anomoura, and macrouru. The braehyura, or short-tailed decapods, 

 the highest group, are first met with in the Cretaceous rocks ; the 

 anomoura in the Jurassic; and the macroura in the Carboniferous. 

 The entire order is on the increa 



The stomapods are scarcely known in the fossil Btate. A single 

 species has been met with in the upper tertiaries of Monte Bolca, 

 and a few doubtful forms in the Jurassic and Devonian strata. 



The amphipods and the laemodipods are aLo rare in the fossil con- 

 dition. The only certain examples are from tertiary beds. 



The marine isopods exhibit fossil examples from the Jurassic forma- 

 tions, upwards. Terrestrial species occur only in amber. 



The xiphosures — including the pterygotus in this order — date from 

 the upper Silurian. They are on the decline. 



