68 ♦ PISCES , CLASSi 



hemi-heterocercy (internal heterocercy, external homocercy, see p. 10) are very 

 common. 



In many Ganoids the "unpaired fins, and sometimes also the paired fins, are 

 fringed on the anterior border with the so-called fulcra — scale-like spines or 

 plates covered with enamel, which are serially arranged and partly overlap 

 one another. All the fin rays consist of two (right and left) halves, and are 

 transversely jointed, usually also branched distally. 



Like the vertebral column, the skull also exhibits very variable degrees of 

 ossification. In the cartilaginous Ganoids this is almost exclusively confined 

 to a number of investing plates on the cranial roof and the basicranial axis. 

 The pterygoid and palatine bones are still fused together ; the mandible and 

 parts of the hyoid apparatus are ossified ; the opercular apparatus is feebly 

 developed. In the Crossopterygii, Heterocerci, and Lepidostei the ossification 

 closely resembles that of the Teleostei, while the- number and arrangement of 

 the several bones agree essentially with the corresponding features in some of 

 the least specialised members of the latter sub-class. 



In the structure of the pectoral arch, and especially of the anterior limbs, 

 the various Orders of Ganoids exhibit considerable difFerences, and represent 

 intermediate links between the Dipnoi, Selachii, and Teleostei. In the 

 Crossopterygii, Chondrostei, and Heterocerci the clavicle still consists of three 

 separate membrane bones ; while in the Lepidostei and Amioidei the lower 

 Segment (infraclavicle) is completely fused with the clavicle. The small 

 structures behind and within the clavicle, corresponding with the scapula, 

 coracoid, and precoracoid remain cartilaginous in the Chondrostei, probably 

 also in the fossil Crossopterygii ; in the remaining Ganoids, on the other band, 

 they are ossified exactly as in the Teleostei. Moreover, with reference to the 

 Position and number of the basal supports in the pectoral fins, most Ganoids 

 agree with the bony fishes ; the earlier Crossopterygii alone possessed a 

 segmented cartilaginous axis, with biserial radialia, as in the Dipnoi. 



Besides the anatomical characters in the dermal covering and the internal 

 skeleton there are still others indicating that the Ganoids are a natural group 

 of fishes. For example, the somewhat elongated muscular base of the principal 

 artery (bulbus arteriosus) is provided internally, as in the Selachii, with several 

 rows of valves, which prevent the return of the blood from the artery to the 

 auricle of the heart. The gills, on the other band, as in the Teleostei, always 

 lie free under an operculum, which usually consists of several bony plates. 

 Again, by the possession of a spiral valve in the intestine, as well as in the 

 structure of the urinogenital System, the Ganoids approach the Selachii more 

 closely than the Teleostei ; on the other band, they all possess an air-bladder 

 opening into the guUet, and also produce numerous small eggs, By the 

 complete decussation of their optic nerves, the Teleostei are distinguished from 

 the Ganoidei, in which the branching optic nerves form a so-called chiasma. 



The Ganoids exhibit their greatest development in Palaeozoic, Triassic, 

 and Jurassic formations ; and from the base of the Cretaceous System upwards 

 they become more and more replaced by the bony fishes. The few still 

 existing Ganoids live either exclus-ively, or at least partly, in fresh water, 

 while the large majority of the fossil forms occur in purely marine deposits. 

 As a rule, only the Tertiary Ganoids occur in freshwater formations. 



The Ganoidei may be divided into five Orders: — (1) Crossopterygii; (2) 

 Chondrostei ; (3) Heterocerci ; (4) Lepidostei ; (5) Amioidei. 



