292 THE OLDEST AIR-BREATHERS 



grey colour, though associated with reddish beds. It contains 

 Zonites priscus as well, though this is very rare, and there are 

 a few valves of Cythere and shells of Naiadites as well as 

 carbonaceous fragments, fronds of ferns, jyigonocarpa^ etc. 

 The Pupce are mostly adult, but many very young shells also ^ 

 occur, as well as fragments of broken shells. The bed is 

 evidently a layer of mud deposited in a pond or creek, or at 

 the mouth of a small stream. In modern swamps multitudes 

 of fresh- water shells occur in such places, and it is remarkable 

 that in this case the only Gasteropods are land shells, and 

 these very plentiful, though only in one bed about an inch in 

 thickness. This would seem to imply an absence of fresh- 

 water Pulmonifera. In the erect Sigillarice. of the second 

 horizon the shells occur either in a sandy matrix, more or less 

 darkened with vegetable matter, or in a carbonaceous mass 

 composed mainly of vegetable debris. Except when crushed 

 or flattened, the shells in these repositories are usually filled 

 w^ith brownish calcite. From this I infer that most of them 

 were alive when imbedded, or at least that they contained the 

 bodies of the animals ; and it is not improbable that they 

 sheltered themselves in the hollow trees, as is the habit of 

 many similar animals in modern forests. Their residence in 

 these trees, as well as the characters of their embryology, are 

 illustrated by the occurrence of their mature ova. One of 

 those, which I have considered worth figuring, has been broken 

 in such a way as to show the embryo shell. 



They may also have formed part of the food of the reptilian 

 animals whose remains occur with them. In illustration of 

 this I have elsewhere stated that I have found as many as 

 eleven unbroken shells of Physa heterostropha in the stomach 

 of a modern Meiiobranchus. I think it certain, however, that 

 both the shells and the reptiles occurring in these trees must 

 have been strictly terrestrial in their habits, as they could not 

 have found admission to the erect trees unless the ground had 



