March 1, 1893.] 



KNOWLEDGE 



55 



a whole may have a motion, of mtiition. All these 

 secondary kinds of motion, which do not consist of motion 

 of the molecule as a whole from place to place, are con- 

 sidered separately and oalleil the internal energy of the 

 molecule. The determination of its nature and amount is 

 a problem of extreme difficulty, and is yet unsolved, 

 though the ratio of the energy of translation or agitation to 

 the internal energy is in some cases known or guessed at. 

 (To be continued.) 



LIVING FOSSILS. 



By E. Lydekker, B.A. 



IN an article entitled " The Oldest Fishes and their 

 Fins," * it was pointed out how that through the 

 discovery of the Australian hmg-fish + (Kjiiri'mtoJuK), 

 the ancient mesozoic fauna of Europe was brought 

 into much closer connection with that of the present 

 day than had hitherto been sui^posed to be the case. 

 This, however, is by no means a solitary instance of the 

 discovery in a living condition of forms of life which have 

 been regarded as long extinct ; and since the subject of the 

 survival of ancient types in remote corners of the earth or 

 the abysses of the ocean is one of wide interest, we propose 

 to consider it La some detail in the present article. For 

 such survivors from a distant past we ventiu'e to suggest 

 the title of " living fossils," seeing that for the most part 

 they have but little in common with the dominant fauna 

 of the greater part of the world ; while their alliance with 

 extinct types is of the most intimate kind. It is of course 

 difficult to know where to draw the line in the use of such 

 an arbitrary designation ; but we shall endeavour to restrict 

 the term either to types which, although still more or less 

 abundantly represented at the present day, are of extreme 

 antiquity, or to such as are now represented by com- 

 paratively few forms, living either in distant parts of the 

 world, or in the ocean depths, but which were abundant 

 in past epochs. Of those coming under the latter category, 

 the majority, as might have been expected, were first made 

 known to science from the evidence of their petrified 

 remains, while their esistmg relatives were not discovered 

 till later. Whether, however, the extinct or the living 

 types were the first to be discovered, the progress of 

 research has been gradually tending to connect the past 

 more intimately with the present than was originally 

 supposed to have been the case. 



Our first examples of " living fossils " will be taken from 

 he mollusca, among which the gastropods, described under 

 the name of Pleurotonuiriii, afford the 

 most striking instance. The shells of 

 this genus, which are frequently very 

 elaborately sculptured, have a general 

 external resemblance to a Turho or 

 a Trochu-s, but are readily distinguished 

 by a deep horizontal slit in the middle 

 of the outer wall of the mouth, from 

 which the genus takes its name. The 

 genus Phurotomaria was originally es- 

 tablished in the year 1826 on the evi- 

 dence of a species from the English lias ; 

 and a host of other extinct kinds were subsequently de- 

 scribed, ranging from the Silurian to the chalk. Till 1855, 

 no one, however, had the least idea that the genus was 



* Knowledgk, September, 1W92. 

 t The writer is indebted to Mr. Davidson for pointing out in tlie 

 .January number of this serial, p. 13, that the name " barramunda" is 

 not applicable to the lung-fish alone, but denotes all the big fisli of the 

 Australian riYors. 



Fig. 1. — Shell of 

 Pleuroiomaria. 

 (After Dr. U. 

 Wiodward.) 



still existing, but in that year a living specimen was 

 obtained otf Mariegalante, which was subsequently sold in 

 London in 1875 for iE25. The species to which this first 

 recent specimen belonged was named P. 'iwiynna .- and 

 three examples of the same form have subsequently been 

 obtained. The next discovery of a I'leumUimaiia occurred 

 in 1861, when an imperfect specimen of another species 

 (/-". iidansonia) was obtained. A second example of this 

 same species was taken in 1882 near Guadaloupe, and three 

 others are in existence, while a seventh was purchased in 

 1890 at Tobago by Mr. K. J. S. Guppy. The latter 

 example was a very large shell, measuring just under six 

 inches in height ; and it was also distinguished by its 

 striking coloration, being marked by a number of oblique 

 splashes of reddish -orange on a pale llesh-coloured ground. 

 A third species of the genus (P. hcijvu-hi), represented of 

 the natural size in our figure, was first obtained from 

 Japanese waters in 1877 ; and three examples have been 

 subsequently secured. Finally, the fourth and lai-gest 

 living species is only known by a single example, which was 

 recognized in 1879 among a collection of shells at 

 Eotterdam, and is believed to have come from the 

 Moluccas. The height of this fine shell is six and three- 

 quarter inches. 



It will thus be apparent that only fourteen specimens of 

 living Pleurotomdria, referable to four distinct species, are 

 at present known to zoologists. These moUuscs, which 

 are known to inhabit deep water on rocky bottoms, must 

 therefore be extremely rare ; although from the nature of 

 their habitat it is probable that not so many specimens 

 are obtained as might otherwise be the case. In the 

 tertiary period, according to Dr. H. V/oodward, only 

 eleven species are known, of which two are from the 

 pleistocene, two from the miocene, and seven from the 

 eocene. Directly, however, we reach the cretaceous, the 

 number of species suddenly leaps up to 208, while the 

 total number of secondary and palaeozoic species is upwards 

 of 1115. Accordingly, out of a total of 1100 representa- 

 tives of the genus, only fifteen are of post-cretaceous age, 

 of which only four now exist, and these apparently poorly 

 represented in individuals. Here, then, we have indeed a 

 striking instance of a " living fossil." 



The well-known bivalve shells named Triiionia afford a 

 scarcely less well-marked case of the persistence of early 

 types. This genus was originally named in 1791, on the 

 evidence of an extinct species, but when fully described by 

 Lamarck in 1804 some few recent living examples had 

 also been obtained. For the benefit of those of our 

 readers who may not be familiar with these molluscs, it 

 may be mentioned that the li^'iug Triijonite, are rather 

 small shells, of about an inch and a half in diameter, 

 characterized by their somewhat triangular shape, and the 

 strongly marked transverse ribs, marked with rough 

 tubercles on the outer surface. Internally the shell has a 

 polychroic pearly lustre ; while the peculiarly-shaped and 

 striated interlocking hinge, when once seen, can never be 

 mistaken. At the present day the Tiijjunuc are represented 

 only by some five closely allied species, confined to the 

 Australian seas ; while in the tertiary, although more 

 widely distributed, they were likewise rare. In the 

 secondary period, where they range down to the lias, 

 these shells were, however, extremely abundant, and 

 attained far larger dimensions than their existing 

 relatives. Indeed, in the oolites Trir/onitr were some 

 of the most common molluscs, whole slabs of rock 

 being sometimes found paved with their handsomely 

 sculptured shells, while all who have visited the Isle of 

 Portland must be familiar with the countless swarms in 

 which casts of their shells occur in the so-called "roach- 



