AIR-BREATHERS OP THE COAL PERIOD. 285 



been found of any of the marine fossils of the lower carboniferous 

 limestone. Abundant remains of fishes occur, but these may have 

 frequented estuaries, streams and ponds, and the greater part of 

 them are small ganoids which, like the modern Lepidosteus and 

 Amia, may have been specially fitted by their semi-reptilian res- 

 piration, for the impure waters of swampy regions. Bivalve 

 mollusks also abound ; but these are all of the kinds to which I 

 have given the generic name Naiadites, and Mr. Salter those of 

 Anthracomya and Anthracoptera. These shells are all distinct 

 from any known in the marine limestones. Their thin edentu- 

 lous valves, their structure consisting of a wrinkled epidermis, a 

 thin layer of prismatic shell and an inner layer of sub-nacreous shell 

 composed of obscure polygonal cells, all remind us of the Anodons 

 and Unios.* A slight notch in front, noticed by Salter, as possibly 

 byssal, concurs with their mode of occurrence in rendering it proba- 

 ble that, like mussels in modern estuaries, they attached themselves 

 to floating or sunken timber. They are thus removed, both in 

 structure and habit, from truly marine species; and if not actually 

 of the family Unionidce, must have been fresh-water or brackish- 

 water mussels closely allied to this family. The crustaceans 

 {Eurypterus^iplostylus^Cyprids^ and the worm shell (Spirorbis^f 

 found with them, are not necessarily marine, though some of them 

 belonged probably to brackish water, and they have not yet been 

 found in those carboniferous beds deposited in the open sea. There is 

 thus in the whole thickness of the middle coal measures of Nova 

 Scotia, a remarkable absence at least of open sea animals ; and if, 

 as is quite probable, the sea inundated at intervals the areas of 

 coal accumulation, the waters must have been shallow and to a 

 great extent land-locked, so that brackish-water rather than marine 

 animals inhabited them. 



On the other hand, there are in these coal measures, abundant 

 evidences of land surfaces ; and sub-aeriel decay of vegetable mat- 



* The microscopic structure of these shells is well preserved, and 

 presents some differences of detail which I hope at a future time to 

 illustrate. 



f The idea of some Palaeobotanists, that these so-called Spirorbes are 

 fossil parasitic plants, is obviously a mistake. They are calcareous 

 shells, and present under the microscope a prismatic cellular structure, 

 with numerous minute tubuli, in the manner of the shells of modern 

 Serpula and Spirorbes. In Nova Scotia I have seen Estherice only in 

 the lower coal formation. 



