280 AIR-BREATHERS OP THE COAL PERIOD. 



being, as is always the case in such, shells, very different in general 

 form from the adults. This bed is evidently a layer of mud deposited 

 in a pond or creek, which afterward became silted up and carried 

 sigillaroid trees. In modern swamps multitudes of shells occur in 

 such places ; and it is remarkable that in this case a single land 

 shell should alone be found, without any trace of aquatic mollusks. 

 The shells which occur in this bed are filled with the surrounding 

 sediment. Those which occur in the erect Sigillarice, on the 

 other hand, except when they are crushed and flattened, are filled 

 with a deposit of brown calc-spar. I infer from this that the 

 latter when buried contained the animals, and consequently that 

 these lived or sheltered themselves in the hollow trees, as is the 

 habit of many modern land snails. 



The gally-worm of the coal period, Xylobius Sigillarice, must 

 have existed in great numbers, as many layers in the erect trees 

 are full of them. It probably lived in these decaying trunks just 

 as its modern congeners do in similar places. As an air-breathing 

 animal, and subsisting on vegetable food, it cannot have lived in 

 water, or even in very wet places ; and this is one of the evidences 

 which in this case point to a greater dryness of the coal swamps 

 than has hitherto been supposed probable ; it also shows the resem- 

 blance of the conditions of the areas of coal accumulation to 

 those of modern forests. 



With regard to the affinities of Xylobius, its form and structure 

 render certain its alliance with the Myriapods, and with the chilog- 

 nathous division of them, or the gally-worms ; but it is less certain 

 to which of the families of the recent gally-worms it belongs, if to 

 any of them. I have however little doubt that if it existed as a 

 recent animal, it would go into the tribe Bizonia of Newport, and 

 probably into the family of Iulidce, to the typical genus of which 

 it bears a strong resemblance in such points as can be made out. 

 The oldest Myriapod previously known is, I believe, the Geophilus 

 proavus, Miinster, of the Jurassic period*. 



EXPLANATION OF PL. VI, FIGS. 4T to 53, and 56 to 61. 



Invertebrate Air-Breathers. 



Fig. 49. — Pupa Vetusta, natural size. 

 " 50. — " " magnified. 



* Pictet, Palaeontologie, Vol. 11, p. 405. 



