1892.] Contributions to the Anatomy of Fishes. 153 



invariable completeness of the encapsulation of the reduced air-bladder 

 by bone in the former may be associated with the fact that both the 

 bladder and the Weberian mechanism otherwise retain their normal 

 structural integrity. 



Geographical Distribution of the Ostariophysece. 



All the Ostariophysese are fresh- water forms, except a few Siluroids 

 which have become accustomed to a marine habitat. Of the known 

 2180 (approximate) fresh-water Teleostei, there are only about 600 

 in which the Weberian apparatus is absent. Broadly speaking, it 

 may be stated that the total number of species of Ostariophyseae is 

 nearly five times as great as all the other species of fresh-water 

 Physostomi ; nearly five times greater than the species of fresh-water 

 Physoclisti; and about three times the number of the species of 

 Physostomi and Physoclisti combined. With the exception of the 

 G-ymuotidae and Gymnarchidae, the families of the Ostariophyseae are, 

 so far as species are concerned, from two to six times as large as the 

 best represented families of the remaining Physostomi or the Physo- 

 clisti ; and the families of the Cyprinidae and Siluridae are by far the 

 richest, both in species and genera, the former including one-third 

 (724), and the latter about one-fourth (572), of all the known fresh- 

 water species. This predominance characterises all the great zoo- 

 geographical regions about which we have any information, with 

 the exception of North America, where other fresh- water Fishes 

 slightly outnumber the Ostariophyseae. In the Indian and Neotropical 

 regions, where fresh-water Fishes attain their maximum degree of 

 specific development, the Ostariophyseae outnumber all the remaining 

 species in those districts in the proportions of 5*5 and 4 to 1 respec- 

 tively. The great rivals to the Ostariophyseae among fresh-water 

 Fishes are the Salmonidae and Cyprinodontidae, but, owing to a differ- 

 ence of habit, or of geographical distribution, a considerable number 

 of their species do not come into direct competition with the former. 



Hence, it may be concluded that the possession of a Weberian 

 mechanism is specially characteristic of the dominant families of 

 fresh- water Teleostei, that is, of those families which combine to the 

 greatest extent numerical strength in individuals, richness in specific 

 differentiation, and wideness of geographical distribution. 



Two other possible conclusions are also suggested by these facts : 

 (1) that the possession of a Weberian mechanism is directly related 

 to certain peculiarities of a fresh-water habitat, and (2) that the close 

 association between the presence of this mechanism and the marked 

 ascendency of the Ostariophyseae over all other families of fresh-water 

 Teleostei points to the possibility that the relation is one of cause and 

 effect. Both conclusions, in our opinion, are highly probable, and the 



