290 AIR-BREATHERS OF THE COAfc PERIOD. 



in aqueous deposits. The erect trees gave it its almost sole chance of 

 preservation. Pupa vetusta is a small species, and its shell very 

 thin and fragile, while it probably lived among thick vegetation. 

 Further it occurs in great abundance in the sigillaria stumps, and 

 also in a bed. separated from these by a thickness of. 1217 feet, 

 including 21 coal seams, having an aggregate thickness of about 

 20 feet, 3 beds of bituminous limestone of animal origin, and per- 

 haps 20 beds holding Stigmaria in situ,or erect Sigillarice and Cata- 

 mites. The lapse of time implied by this succession of beds, many of 

 them neccessarily of very slow deposition, must be very great, though 

 it would be mere guess work to attempt to resolve it into years. 

 Yet long though this interval must have been, Pupa vetusta lasted 

 without one iota of change through it all ; and more remarkable 

 still, was not accompanied by any other mollusk of its family. 

 "Where so many specimens occur, and in situations so diverse, 

 without any additional species, the inference is strong that no 

 other of similar habits existed. Tf in any of those sub-tropical 

 islands, whose climate and productions somewhat resemble those 

 of the coal period, after searching in and about decaying trees, and 

 also on the bars upon which rivers and lakes drifted their bur- 

 dens of shells, we should find only a single species, but this in 

 very great numbers, we would surely conclude that other species, 

 if present, were very rare. 



Again, footprints referrible to Dendrerpeton occur in the lower 

 coal measures below the marine limestones, in the middle coal 

 measures, and in the upper coal formation, separated by a thick- 

 ness of beds which may be estimated at 15,000 feet, and certainly 

 representing a vast lapse of time. Did we know the creature by 

 these impressions alone, we might infer its continued existence for 

 all this great length of time ; but when we also find its bones in 

 the principal repositories of reptile remains, and in company 

 with the other creatures found with it, we satisfy ourselves that of 

 them all it was the most likely to have left its trail in the mud flats. 

 We thus have reason to conclude that it existed alone during 

 this period, in so far as its especial kind of habitat was con- 

 cerned ; though there lived with it other reptiles, some of which, 

 haunting principally the woods, and others the water, were lesa 

 likely to leave impressions of their footprints. These may be but 

 slight indications of truth, but they convey strong impressions of 

 the persistence of species, and also of the paucity of species belong- 

 ing to these tribes at the time. 



