176 



HARDWiCKE'S SCIENCE- GOSSIP. 



this meaus twenty-six pages would be employed 

 instead of one, yet the subdivisions would be so 

 much smaller than by the usual method that the 

 extra labour would be fully compensated for. Thus 

 the heading for the first page would be aa, ab, ac, 

 ad, &c., and for the second ba, bb, be, bd, &c. 

 Notwithstanding some columns, such as bb, would, 

 most probably, not contain a single entry, yet I 

 should retain them, for the eye soon becomes 

 accustomed to the position of the column for each 

 letter, and knows at a glance the spot where the 

 entry should be made. 



If you think this plan will help Mr. Mott and 

 others, I hope you will find room for it ; if not, 

 please consign it to the waste-paper basket. 



Alex. E. Mtjrkay. 



CARBONIFEROUS EISH. 



A T the base of the coal-measures there occurs in 

 -^-^ Yorkshire a bed of coal which is different 

 from any of those above it. This bed — the Halifax 

 bed — is covered by a "roof" unlike that of any 

 other coal-bed above the mountain limestone, and 

 consisting of nodular concretions called "baum- 

 pots," which contain not only the remains of plants 

 and fresh-water shells, but also of fish and marine 

 shells. The floor or sill of the coal of this series, in- 

 stead of being, as in the other series, a peculiar fine 

 clay, full of stigmaria, consists of ganister or gal- 

 Uanl, a hard siliceous sandstone full of the roots 

 of plants. The presence of this stone consequently 

 furnishes an exact iudication of the position occu- 

 pied in the coal-measures of that stratum in which 

 it is found. 



In the " baum-pots " before mentioned I dis- 

 covered last July, when staying in the neighbour- 

 liood of Leeds, several remains of fish, both scales 

 and teeth, belonging to Agassiz's two great orders 

 of fish, the Ganoids and Placoids. 



The ganoid fish have been thus named by him 

 on account of their being covered by enamel 

 so hard that, if struck with steel, they will emit 

 sparks like flint ; thus differing from the fish belong- 

 ing to other orders. Another peculiarity attaching 

 to them is this, — the union of reptilian with fish- 

 like characters, they being able to move the head 

 upon the neck independently of the body, and the 

 connection of the vertebrae by ball-and-socket joint, 

 instead of by inverted cone, as in the ordinary 

 fishes. 



The representatives of this order of fishes are the 

 Lepidosteous, or bony pike, and the Amia of North 

 America, the Folypterm of the Nile and rivers of 

 Senegal, and the Ceratodus of Australia. As all 

 these genera inhabit fresh water, analogy points to 

 the conclusion that their carboniferous ancestors 

 lived in the same element. 



No doubt many of those who have found, the 

 teeth and scales of extinct ganoids have been puzzled 

 by not finding their entire skeletons. This is owing 

 to these fish not having possessed entire bony skele- 

 tons : consequently,' the cartilaginous portions of 

 the fish have perished, whilst the teeth and scales 

 have been preserved. In Megaliclithys, however. 

 Professor Huxley has discovered a ring or hoop of 

 bone in its vertebra, whilst the rest of its skeleton 

 consisted of cartilage. 



Fig:. 110. Ganoid Scale of Megalic)ithys. 



Of the scales of these fish examples are given 

 That of Megalichthys Hibberti (6g. 110) is of a 

 smooth rhomboidal form, with minute punctures 

 that are connected with one another by means of 

 canals. 



The other scale, that of Acrolepis (figs. 111 and 

 112), is similar to the foregoing in shape, but differs 

 from it in having its surface marked by deep chan- 

 nels. The former genus is found only in the car- 

 boniferous system, whilst the latter is found both 

 in the carboniferous limestone of Derbyshire, the 

 millstone grit and coal-measures of Yorkshire 

 and in the Permian of Duihani. 



Fig-s. Ill antl 112. .Scales o( Acrolepis. 



The other great order of fishes, the Placoids, 

 represented in our time by the Sharks, Dogfish, 

 and Rays, have been so named by Professor Agassiz 

 because they have their skins covered irregularly 

 with plates of enamel, often of considerable dimen- 

 sions, but sometimes reduced to mere points, like 

 the shagreen on the skin of the Shark or the prickly 

 tubercles of the Ray. The fishes of this order had 

 cartilaginous skeletons, which were soon destroyed 

 after they had perished. 



The teeth of these fish are of tico kinds, — either 

 sharp and pointed, or massive palatal teeth, fitted 

 for grinding. Of both these we have living ex- 

 amples ; those of the Shark being representative o f 

 tlie former, and those of the Cestracion, the Port 

 Jackson shark, of the latter. 



5#^ 



Fig-. 1 13. Tooth of Cladodus. Fig. 114. Tooth of Orodtts 



