74 MONOGRAPH OF DURA DEN. 



where found in the whole composite mass, nor in any one of 

 the numerous quarries in the district. 



2. What constitutes a geological formation, and how are the 

 limits to be determined 1 Here, in the yellow sandstone alone, 

 the fossil remains consist of 7iine genera and of eleven species, 

 namely : Pamphractus Andersoni (pterichthys), Holoptychius 

 Andersoni and Flemingii, Platygnathus Jamesoni and Minor, 

 Glyptopomus Minor, Glyptoljemus Kinnairdi, Phaneropleuron 

 Andersoni, Diplopterus Dalgleisiensis, Phyllolepis Concentricus, 

 and Dendrodus (?). Two of the genera are common to the Old 

 Red and the Carboniferous systems, Holoptychius and Diplo- 

 pterus. Three of the genera are found in the Lower and the 

 Upper series of the Old Red, Pterichthys, Platygnathus, and 

 Diplopterus. Three genera are common to the Middle series 

 of Morayshire and Clashbennie, and the Upper series of Dura 

 Den, Dendrodus, Phyllolepis, and Diplopterus. 



The determination of a geological formation or system, 

 according to this list, cannot be made exclusively to depend 

 upon the specific, nor even upon the generic forms of organic 

 remains included in a series of rocks. The holoptychius and 

 diplopterus belong to two different formations, and pterichthj^s, 

 platygnathus, and diplopterus range from the lowest to the 

 highest beds of the Old Red strata. Will independent suites 

 of rocks, included in the same or in different formations, serve 

 for the determination of specific distinctions ^ Suppose a dis- 

 position to exist among palceontologists to merge four species 

 of holoptychius into one and the same, namely, Holoptychius 

 Murchisoni, nobilissimus, Andersoni, and Flemingii, what amount 

 of consideration would be due to their several positions in the 

 systems in which they are respectively included ? M. Agassiz 

 asserts that he does not know a single species of fossil fish 

 which is found successively in two formations, while he is 

 acquainted wuth a good number wdiich have a very considerable 

 horizontal extent.* 



* Edinhuryh Philosophical Journal, p. 175. 



