TELEOSTOMES  , members of the third sub-class of the class Fishes, being all the fishes in which the

skull is invested with membrane bones, viz., the Crossopterygians, the Dipnoans, the Ganoids and the Teleosteans . They may be further defined as fishes with an ossified or cartilaginous skeleton, a lower jaw, gills inserted on the gill-arches, a single gill-opening on each side (exceptionally fused with its fellow on the ventral side), an opercle formed of one or several bones, the body usually covered with scales or bony plates, an air-bladder or lung, at least in the primitive forms, and without copulatory paired organs or " claspers." The term which designates this sub-class has been adopted by Sir R . Owen, E . D . Cope, and A . S . Woodward in a less comprehensive sense, the Dipneusti being regarded by them as constituting a separate stfb-class, and its inventor, C . L . Bonaparte (1838) had proposed it in a more restricted sense, the sturgeons, lophobranchs and plectognaths being excluded . T . Gill (1872) was the first to use it in the acceptation taken in the present article . Whether the Ostracophores should be included among the Teleostomes, as recently proposed by C .

T . Regan, is still open to doubt . The sub-class is here divided into four orders, but it is difficult to decide whether, in an ascending

series, the Crossopterygians or the Ganoids should be placed first . From the point of view of the evolution of the paired fins, accepting the lateral fin-fold theory as the better supported by the evidence at hand, there is much to say in favour of regarding the Chondrostean Ganoids as the more primitive type . From another point of view the condition of the air-bladder in the existing Crossopterygians appears to represent the earliest form assumed by this important organ, which it seems rational to conclude was originally evolved asan accessory breathing organ and later became transformed into a hydrostatic apparatus (Ganoids and Teleosteans) on the one hand, into a true lung (Dipnoans and Batrachians) on the other . Guided by the second consideration, assuming that the air-bladder of the fossil Crossopterygians conformed to the type known in their recent representatives, and also in deference to palaeontological chronology, whatever it be worth in the present state of our knowledge, we shall begin the series with the Crossopterygians, which pass into the Dipnoans, and then take up the Ganoids, which lead up very gradually to the Teleosteans, the dominant group at the present day . But we do not deny the force of the arguments adduced by Regan in attempting to show that the paired fins of the Chondrostean Ganoids are a nearer approach to the primitive condition than are those of the Crossopterygians . No doubt some day we shall become acquainted with still older Teleostomes, ,which we may expect to establish the connexion between the two types which in Palaeozoic times have evolved on parallel lines .