HARDWICKE'S SCIENCE-GOSSIP. 



123 



form into groups, which finally become developed 

 into organs. Now the shell is seen to form as a very 

 thin layer over one portion of the embryo, and, grow- 

 ing, takes a nautiloid form. Into this shell the whole 

 of the embryo soon contracts itself, and meantime the 

 differentiation of cells into organs proceeds. By the 

 time the embryo is completely withdrawn into its 

 shell, the stomach, liver, and intestines are clearly 

 visible, so also is a large muscle which attaches the 

 embryo to the shell. It has now the power of move- 

 ment by the alternate contraction and dilatation of its 

 broad " foot," and it continues to perambulate the 

 egg-sac until the rupture of the latter sets it free. 

 But although its form and power of movement might 

 lead us to believe that it is a complete snail, this is 

 not the case. As yet it is totally devoid both of 

 nervous and circulating systems. These, however, 

 rapidly follow, and our little snail is full-fledged. It 

 is at once independent, and roams about over the 

 surfaces of plants and stoneS; cropping the tiny 

 growths of algae, and the leaves of larger water- 

 plants. 



The figures given in our last number represent a 

 series of stages in the development of the embryo, 

 sketched from the microscope. It may be interesting 

 to note, that what is known as the "foot" of the 

 snail is shown by this embryonic development to be 

 really an under-lip. 



A SKETCH OF THE FLORA OF THE COAL 

 PERIOD. 



By Rev. J. Magens Mello, M.A., F.G.S., &c. 



TO form a true idea of the flora of any one of the 

 past geological periods is an undertaking of 

 no easy nature, and the task set before a palaeonto- 

 logical botanist has been vividly described by Unger. 

 " Imagine," he says, " the remains of a great forest, 

 rich in trees and bushes of all species, in which all 

 would be thoroughly commingled, confounded and 

 heaped up together, twigs, branches, leaves, buds, 

 bracts, scales, flowers, fruits, seeds, "of the most varied 

 sorts, as well as pieces of bark and wood ; and this 

 chaos of vegetable matter will hardly give an idea of 

 that which the palaeontologist has to unravel in order 

 to recover the hidden meaning, and restore all to its 

 natural aspect, in a word to reproduce each plant as 

 it was when living." Besides aU this, the very 

 remains we have are themselves for the most part in 

 a very imperfect and fragmentary condition. An- 

 other point which must be borne in mind in endeav- 

 ouring to restore an extinct flora is that the majority 

 of the more herbaceous plants have left hardly a 

 trace of their existence, although they must doubtless 

 have been abundant, and in all probability have borne 

 the same ratio to other plants that they do at present. 

 A great source of difficulty is the fact that it is not 



always easy to describe a plant from even such a 

 perfect fragment as a well-preserved leaf, or piece ot 

 its wood. Many different families have very similar 

 leaves, which even in a living state would be practi- 

 cally undistinguished from one another ; the broad 

 division between endogens and exogens, or between 

 these and cryptogams can indeed be made out, but 

 very little else. It is pretty nearly the same with 

 the woody structure : some families, such as the 

 Conifera^, can be recognised by the character of the 

 wood, and, as in the case of leaves, the distinction is 

 readily made between the greater divisions of the 

 vegetable kingdom ; but to say that such and such 

 sections of wood belong to this or that tree respec- 

 tively, is more than we have as yet arrived at, what- 

 ever may eventually be the case. Were the flowers 

 of the various plants frequently met with in a fossil 

 state we should perhaps have a somewhat easier task, 

 but these are rare indeed. Fruits and seeds are found 

 in greater or less abundance in certain beds, and these 

 are sometimes sufficiently well preserved to enable 

 us to say something as to the nature of the plant to 

 which they belonged. 



The Carboniferous Flora. 



Previous to the Tertiary period, the vegetation of 

 which has to a large extent been recovered, the rocks 

 of the coal measures have yielded the most abundant 

 collections of vegetable remains preserved in the 

 earth's crust. 



It has now been long known that all ordinary coal, 

 whether found in rocks of the Carboniferous age, or 

 in those of later date, betokens an extraordinary 

 development of plant life under highly favourable 

 conditions, and that this coal has been formed by a 

 series of chemical changes, carried on through an 

 incalculable period of time ; by which the life of 

 former ages has been transformed by a process of 

 condensation of the carbon, the stages of this conden- 

 sation being in part shown by the gradation of peat 

 into Lignite, and thence to Cannel, Bituminous and 

 Anthracite coals. 



To produce the vast thickness of vegetable matter 

 constituting some of our coal seams there must have 

 been required the successive growth and decay of 

 many thousands of years, and judging from the nearly 

 uniform nature of the coal formation and its flora, in 

 widely separated areas, it would appear that there 

 must have been nearly uniform, although not neces- 

 sarily simultaneous conditions prevailing over the 

 greater part of the earth's surface. 



The remains of the Carboniferous flora are found 

 preserved in sandstone and shale, and in the coal 

 itself. Many years ago. Professor Goppert is said to 

 have detected in beds of pure coal remains of plants of 

 every family then known to occur fossil in the coal 

 measures. 



Climate. — The fossil flora of the various epochs 



G 2 



