66 PISCES CLAS.s I 



present. Fairedßns well developed ; tau dipht/cercal, heterocercal, or hemi-heterocercal. 

 In the living forms — optic nerves not completely decussating but forming a chias7na, 

 biUbus aiieriosus of the heart with numerous valves, intestine with a spiral valve, and 

 air-bladder connected by a tube with the gullet. 



When founding the " Order " of Ganoids, Agassiz paid attention exclusively 

 to the dermal skeleton, and included under this denomination all fishes with 

 scales which consisted of a bony lower layer and a superficial covering of 

 enamel. 



Johannes Müller first attempted to define the Ganoids from anatomical 

 characters. After the removal of the Plectognathi, Lophobranchii, and Siluroids 

 recognised as true bony fishes, and after the assignment of Lepidosiren to the 

 Dipnoi, there still remained as true Ganoids a large number of recent and fossil 

 fishes, which, according to Johannes Müller, formed a distinct sub-class between 

 the Selachii and the bony fishes. While the condition of the muscular bulbus 

 arteriosus, the incomplete decussation of the optic nerves, and the spiral valve 

 in the intestine agree with the former, the arrangement of the head bones, the 

 structure of the fins, and usually also the ossification of the skeleton, are 

 suggestive of the bony fishes. After C. Vogt had recognised in the 

 anatomical structure of Amia a true Ganoid, with thiii elastic cycloid scales 

 and a completely ossified skeleton, the line of demarcation between the 

 Ganoidei and Teleostei seemed to have been absolutely destroyed. Kner, 

 ThioUiere, Owen, and more recently Cope and Smith Woodward have ventured 

 still further, suppressing the Ganoidei as a separate group, and uniting its 

 members with the Teleostei under the general denomination of Teleostomi. 



The most conspicuous, even if not the distinctive featnre of the Ganoids, 

 consists in their dermal covering. No other sub-class of fishes possesses scales 

 which consist of a thick bony inner layer and an outer enamel layer. In no 

 other group is there a union of scales by peg-and-socket articulations, such as 

 is characteristic of the rhombic-scaled Ganoids. There are, however, ganoid 

 scales of rounded form, which overlap each other exactly like cycloid and 

 ctenoid scales, and which are sometimes not perceptibly thicker than the 

 latter ; but these rounded scales (Caturus, 3Iegalurus, Macrarhipis, Amia) exhibit 

 beneath the enamel covering an inner layer with bone cells, though sometimes 

 its thickness is excessively reduced. The Spatularias are characterised by 

 the complete absence of scales, except on the upper lobe of the tail ; in the 

 Sturgeons the body is partly covered with large bony plates. 



The structure of the internal skeleton of the Ganoids is of special interest, 

 notably that of the vertebral column. In sonie Chondrostei (Acipenser) the 

 vertebral axis, with its arches and spines, remains completely cartilaginous ; but 

 in Spatularia, the older Crossopterygii, almost all Heterocerci and Pycnodonts, 

 and part of the Lepidostei, there is a partial or complete ossification of the 

 arches, spinous processes, and supports of the median fins, this beginning at 

 the surface and extending inwards, so that ä cartilaginous core is usually 

 surrounded by a hollow bony cylinder. These are termed " naked vertebrae " 

 {Nacktwirbel), (Fig. 120), in contradistinction to the so-called " hemi-vertebrae " 

 {Halbwirbel), and "ring vertebrae" {Hohlwirbel). In the first (Fig. 121) there 

 develops below the notochord a horseshoe-shaped bony plate (hypocentrum, 

 intercentrum), to which the haemal arches in the caudal region are fixed ; the 

 upwardly directed side-pieces of these hypocentra are mostly attenuated and 

 pointed. The vertebral centrum proper is represented by a pair of lateral 



