108 PALEONTOLOGY. 



hypothesis, in the present family, to the soft-scaled covering 

 of later fishes. 



The lepidoganoid type of fish is not known to have existed 

 earlier than the Devonian period ; at which, however, it 

 already offered two forms of the overlapping scales, Dipterus 

 shewing the rhomboid, and Holoptychius the rounded form. 

 Of the lepidoganoids of the carboniferous strata, Pcdceoniscus, 

 Pygopierus, Acrolepis, Eurynotus, Eloniclithys, Phctrolepis, 

 Graptolepis, OrognatJius, Poclodus, Acanthodes, and Diplopterus, 

 had rhomboid scales. Gcelacanthus, Isodus, Phyllolepis, Hop>lo- 

 pygus, Uronemus, Golonoclus, Centrodus, Asterolepis, Psarn- 

 mostcus, and Osteoplax, had rounded scales. 



Of the above-named genera, Acrohpis, Pygopterus, Pcdceo- 

 niscus, and Gcelacanthus, continue to be represented in Permian 

 beds ; in which also are found species of the ganoid genera 

 Dorypterus, and Globuloclus, if the teeth on which the latter is 

 based be not those of Platysomus, a pycnodont genus which is 

 both Permian and carboniferous. 



The formations of the mezozoic or secondary periods give 

 evidence of the full development of the ganoid order. In the 

 lowest or " triassic " division, this order is still represented by 

 heterocercal and notochordal species belonging to some of the 

 genera of the Permian period, as, e.g., Gcelacanthus, Amhly- 

 pterus, and Pcdceoniscus. The genus Placodus, a supposed 

 pycnodont fish of the muschelkalk, has been shewn to be a 

 conchivorous Saurian. 



Of 33 genera of fishes in the lias, 4 only were repre- 

 sented in older strata, while the rest extend into the upper 

 oolitic beds. Most of these are Ganoids with rhomboid 

 scales. Leptolejns has rounded scales ; and this shape be- 

 comes more common in the mezozoic genera which appear 

 later than the lias, such as Thrissops, Mcgcdurus, Oligop)hurus, 

 etc. 



