THE SILURIAN PERIOD. 417 



breathing pores, and hence to be land species, as are the present forms, 

 but this has been questioned. The sting and the poison glands have 

 been identified, and the significant name Palceophonus, " ancient mur- 

 derer," applied in consequence (Fig. 189, c). Whitfield, however, thinks 

 that the American species may have been aquatic. 



The presence of fishes. — The addition of fishes emphasizes the 

 pecularities of this fauna. Except for their singular occurrence in Colo- 

 rado in the Ordovician, fish remains have not been found in America 

 until this stage. Palceaspis americana (Fig. 189, d), a low fish-like 

 form referred to the ostracoderms, has been described by Claypole 

 from Pennsylvania. In Europe a few fishes appear somewhat earlier 

 than this in the Ludlow formation, but nearly all the fish remains of 

 the period occur in the very highest horizons regarded as Silurian, 

 or in the deposits that form the transition to the Devonian period, 

 where they are associated with eurypterids and land plants, as well 

 as marine invertebrates. It would appear from the remains that 

 fishes were rather abundant and varied in their development at cer- 

 tain stages. The relics, however, are fragmental and embrace teeth, 

 dermal plates, spines, shagreen scales, etc. These fishes appear to 

 have been the forerunners of the abundant fishes of the Old Red Sand- 

 stone (Devonian) into which the transition beds of Great Britain gradu- 

 ate, and as these Old Red Sandstone beds were probably land-water 

 deposits, the association of the fish with the eurypterids and with 

 land plants carries some further presumption that the peculiar crusta- 

 cean fauna lived normally in land waters. It is not to be overlooked, 

 however, that in the transition beds the fishes are also associated with 

 marine fossils, and there is no question that, before the close of the 

 Devonian period, certain fishes at least became truly marine. It is 

 therefore entirely consistent with either view of the place of origin 

 of fishes to regard these as marine forms, or as forms frequenting both 

 salt and fresh water. It is to be noted further that not a few of the 

 forms conveniently referred to here as fishes were probably ostraco- 

 derms which have certain arthropod characters and are not unlikely 

 to be made a separate class, as will appear later. 



