616 J. W. DAVIS ON REMAINS OF FOSSIL FISHES 
reaches its maximum, and in the upper beds there are one or two 
thin seams of coal. 
The inference to be drawn from these facts appears to be, that, 
during the period when the thick Mountain Limestone was in process 
of formation in the southern area, water constantly occupied the 
space, and whilst the ground was lowering and the limestone 
increasing in thickness in a more or less gradual and regular 
manner, a very much more complicated series of changes was taking 
place in the district to the northward. The alternations of 
limestone, shale, and sandstone, indicate repeated changes in the 
level of the land. At one time the water was deep enough for the 
existence of the animal forms of life which originated the limestone ; 
at others, the mud, brought from some adjacent land, formed 
accumulations represented by the shales, probably in water shallower 
than during the formation of limestone. The sandstones point to 
extensive shore-deposits and the immediate proximity of land, 
whilst the coal-seams are the result of the decomposition of 
plants which grew on the land, the disintegration of which 
supplied the materials for the accumulated deposits of shale and 
sandstones. 
The fish-fauna preserved in these strata presents some peculiarities 
which distinguish it from that of the thick-bedded Mountain Lime- 
stone below, as well as from that of the Coal-measures above. The 
remains of large predaceous fishes common in the Lower Limestone, 
such as Ctenacanthus major, Phoderacanthus, and the large species of 
Orodus, are absent ; on the other hand, the similarly large represen- 
tatives of the Coal-measures, such as Gyracanthus and Ctenacanthus 
hybodozdes, are also absent from the Red Beds of Wensleydale. Several 
genera found in these beds are, however, common to the Mountain 
Limestone, such as Petalodus, Lophodus, and Cladodus ; but they are 
for the most part of different species. Polyrhizodus, Psammodus, 
Copodus, and Pacilodus, typical forms in the Mountain Limestone, 
are also represented ; but the specimens hitherto discovered have been 
comparatively small, and they appear to be almost, or quite, the last 
representatives of their respective genera just on the point of extinc- 
tion. Their remains do not occur, so far as present evidence exists, 
higher in the series. The Coal-measure genus Plewrodus appears 
for the first time in Yorkshire strata in the Red Beds; and the 
Ganoid fish Megalichthys appears to be represented by a small 
jaw found in the limestone of Wensleydale, probably its first 
appearance. In addition there are numerous genera which, so far 
as at present known, are peculiar to these beds, and which do not 
possess any distinctive character in common with those either of the 
Limestone or the Coal-measures. 
All these circumstances taken together appear to indicate that 
the fauna characteristic of the older limestones was gradually 
becoming extinct; that some species, only previously found in the 
Coal-measures, already existed during the deposition of the Yoredale 
rocks; and that there was also a considerable number of new forms 
which now appear for the first time, and, so far as present observa- 
