28 



KNOWLEDGE 



lFebkuaey 1, 1901. 



leaves and stems arrange themselves with respect to 

 the direction of the source of light. Climbing plants 

 are, however, but little affected by the direction of the 

 source of light. If they were, they would frequently 

 grow away from the structures which support, them, 

 which would be fatal to their continued climbing. Many, 

 on the conirary, exhibit a marked apheliotropism, or 

 turning away from the source of lighit. The Ivy, for 

 instance, in its growth keeps the tip of its shoot pressed 

 against the wall or trunk to which it clings, and will 

 follow the supporting surface into the darkest nook. 

 And climbei-s like the Bryonies and Vetches will grow 

 right up through the dark centre of a hedge, while the 

 surrounding plants bend away from the hedge, seeking 

 the light on either side. 



LIVING MILLSTONES. 



By R. Lydekker. 



The mill-like action of their own upper and lower molar 

 teeth upon one another may have been quite sufficient 

 to suggest to our prehistoric parents the idea of opposing 

 3 pair of corrugated stones in such a manner that by 

 mutual rotation or revolution they should be capable 

 of reducing to powder hard substances placed between 

 them. Indeed the idea of millstones is such a simple 

 and natural one that it is quite probable it may have 

 occurred to the human mind without reference to any 

 prototype in nature ; and, in any case, if such a natural 

 prototype is to be sought, it is not necessary to go 

 further in search of it than our own dental organs. 

 Excellent, however, for their special purpose as are thes" 

 organs (when not subject to prematvire decay), there 

 are other types of tooth-structure to be met with in the 

 animal kingdom which present a much closer approxi- 

 mation to millstones, and might well have foreshadowed 

 these instruments, had they only been accessible to the 

 primeval savage. But since these natural millstones 

 occur only in marine fishes, some of which inhabit 

 distant seas, while others are met with only as fossils 

 deeply buried in the rocks, it is evident that the idea 

 of artificial millstones is not derived from these 

 natural prototypes. In other words, to use an expression 

 now fashionable in natural science, the development of 

 artificial and natural millstones is a case of parallelism. 

 In spite of the fact that their early ancestors were 

 provided with a good working set of sharply pointed 

 dental organs, birds in these degenerate days manage 

 to get along without teeth at all. A few mammals, too, 

 like the South American anteaters, are in the same 

 condition ; and some people have thought that in a 

 few moi-e generations civilised man himself will be 

 reduced to the same toothless state. The great majority 

 of mammals, however, possess a more or less efficient 

 set of teeth, varying in shape, size, and number accord- 

 ing to the need of each particular species or group. 

 But there is one feature common to these organs in 

 mammals of all descriptions ; and this is that they arc 

 strictly confined to the margins of the jaws, never 

 extending either on to the palate, or to the .space en- 

 closed between the two branches of the lower jaw. In 

 many reptiles, such as crocodiles and a large number of 

 lizards, tlie same law of dental arrangement obtains. 

 In some lizards, and still more markedly in certain 

 extinct members of the reptile class, we find, however, 

 a number of teeth developed on the palate, having 

 flattened crowns, and thus tending to make the mouth 

 act the part of one large millstone. But we must descend 



a stage further in the sca.le of animated nature before 

 we come to structures which are strictly comparable 

 with artificial millstones and crushing cylinders. And 

 it is in the class of fishes that we meet with these organs 



1. 2. 3. 



Fig. 1. — A Dental Plate of a Beaked Eagle-Bay {Ehinoptera). 



Fig. 2. — Imperfect Dental Pktes of a Paleeozoic Shark (CocliUodns). 



Fig. 3. — Some of the Lower Crushing Teeth of an Enamel-scaled 

 Fish [Cceloilus). 



in the full perfection of this type of development. Not 

 that they occur by any means in all the groups of that 

 class; the fact being that at the present day living mill- 

 stones are going out of fashion, the great ijreponderance 

 of modern fishes having their dental armature mainly 

 restricted to the margin of the jaws, with or without a 

 minor development of crushing teeth on the palate or 

 the bones of the gullet. With the exception of a com- 

 paratively limited number of cases, showing a different 

 type of development, to which it is not my present 

 intention to allude, these dental millstones are confined 

 at the present day to those hideous marine fishes com- 

 monly known as skates and rays, and to the singular 

 Port Jackson shark and a few allied species inhabiting 

 the Pacific aad Malayan seas. On the other hand, 

 the seas of the Cretaceous, Jurassic, and antecedent 

 epochs absolutely swarmed with numerous kinds of 

 sharks, more or less nearly related to the Port Jackson 

 species, whose mouths were filled with pavements of 

 teeth showing marvellous variety of structure and beauty 

 of oi-namentation. The skates and rays, too, displayed 

 types of dental millstones quite unlike any of those of 

 the present day. And in addition to these, there were 

 hosts of enamel-scaled fishes whose mouths were likewise 

 crammed with beautiful crushing teeth, albeit of a totally 

 different type of structure to that obtaining in either the 

 sharks or the rays. Although well nigh extinct, these 

 enamel-scaled fishes are still represented by the bony pike 

 of the rivers of North America, and the bichir (remark- 

 able for its fringed fins and the row of finlets down its 

 back) of tropical Africa. But it is noteworthy that in 

 neither of these survivors of an ancient group do we find 

 the mouth furnished with an apparatus of millstones ; 

 while, as already said, among the host of sharks that 

 infest the warmer seas of the globe it is only in the Port 

 Jackson species and its three kindred that we find similar 

 structures retained ; all the other members of the group 

 having developed cuspidate teeth adapted for seizing 

 and tearing soft-flcshed prev, instead of for grinding up 

 mail-clad food. 



Clearly, then, there has been some general cause at 

 work which has rendered crushing teeth, so to speak, 

 unfashionable among the fishes of the present day and 

 the immediately antecedent epochs. And in this con- 

 nection it is important to notice that there has been 

 an even more strongly mai'ked tendency to the ex- 

 tinction of the enamel-scaled fishes, and their replace- 



