392 MEMOIRS OF THE CARNEGIE MUSEUM. 



Subclass. Order. Suborder. 



Crossopterygii Actinistia. 



Chondrostei. 



Holostei. 



Teleostei. 



Teleostomi 



After these brief introductory remarks we pass on to the enumeration of the 

 genera and species of the fishes from Solenhofen which are represented in the 

 collections of the Carnegie Museum. 



CLASS PISCES. 

 Sub-Class I. ELASMOBRANCHII. 



"The Elasmobranchs are certainly a very primitive race of Fishes. Their 

 earliest representatives of whose structure we have any precise knowledge (e. g., 

 Cladoselache and Pleuracanthus) are in many respects the most archaic of known 

 gnathostomatous Craniates, and from such types as these, among others, we 

 may very reasonably look for the ancestors of all or most of the remaining groups 

 of Fishes. It has been well said of Pleuracanthus that 'it is a form of Fish which 

 might with little modification become either a Selachian, Dipnoan, or Crossop- 

 terygian," while the condition of the primary upper jaw in the Chondrostean 

 Polyodon suggests that even the more primitive Actinopterygii had an Elasmo- 

 branch origin." (Cambridge Natural History, Fishes, pp. 435-6.) 



We here follow the arrangement adopted by Professor T. W. Bridge in the 

 Cambridge Natural History, grouping the Selachians and Batoidei under the 

 ordinal term of Plagiostomi. It should be borne in mind, however, that authorities 

 are not agreed as to the propriety of maintaining these subdivisions in the ordinary 

 manner, that is, classifying as sharks those cartilaginous fishes which have lateral 

 giU-clefts, and as rays those with ventral gill-clefts. More or less constant differ- 

 ences exist with regard to the manner of specialization of the vertebral centra, 

 as indicated by the terms asterospondylic and tectospondylic, and it is further 

 recognized that modern sharks and rays form two approximately natural groups, 

 the former tending towards agility in swimming, the latter towards expertness in 

 feeding on the bottom. Although a few existing sharks have become adapted 

 for hfe on the sea-bottom and have a depressed form of body, nevertheless they 

 do not have the enlarged pectoral fins which belong to the rays, and the anal fin 

 in no case disappears, as it does in the latter. 



» Smith Woodward, Vert. Palffiont., 1898, p. 32. 



