68 



NA TURE 



\Nov. 27, 1873 



and has introduced us to the fourth and last Order be- 

 longing to the frog's class of vertebrate animals. The 

 Labyrinthodonts were creatures with long tails and mostly 

 two pairs of limbs, but these members were ahyays 

 relatively small with slender toes. Some species attained 

 a greater size by far than does any existing Urodele, even 

 the gigantic Salamander. 



To what existing animals can these huge monsters be 

 considered to have affinity ? It is impossible to say that 

 they in any way bridge over the chasm separating the 

 Frogs from the Efts. They appear indeed to have been 

 almost equally removed from both— for the possession of 

 short limbs and a long tail (characters common to so 

 many widely different animals) cannot be regarded as any 

 good evidence of affinity. 



It is not improbable that they find their nearest aUies 

 in the e.xisting insignificant Ophiomoypha. The latter, 

 though apparently naked, have minute scales imbedded 

 in the skin and arranged in rings at intervals, and the 

 skull is provided with certain e.xtra ossifications. The 

 Labyrinthodonts have similar extra cranial ossifications, 

 and though they have not rings of scales, the ventral 

 region was protected by minute plates arranged in linear 

 series converging inwards and forwards towards the 

 middle line. Moreover, some forms appear to have been 

 entirely devoid of limbs ; at least no remnant of such parts 

 has yet been discovered. Nevertheless the degree of 

 development of the tail constitutes a marked distinction 

 between the Lahyrinfhodoiita and the Ophiomorpha. 



Certain Labyrinthodonts had great formidable teeth in 

 elongated jaws like those of crocodiles. Altogether these 

 singular remains tempt us to speculate as to the succes- 

 sion of life upon this planet's surface. We know that as to 

 the later secondary period that part in the life of the globe 

 which is now played by beasts was then played by 

 reptiles. Instead of the existing bats, Pterodactyles of 

 all sizes flitted through the air. The ocean was peopled 

 not by whales and dolphins, these had not yet appeared, 

 but by huge Ichthyojauri and Plesiosauri. Reptiles of 

 huge bulk (Iguanodons, Mcgalosauri, Notosauri, &c. &c.) 

 fulfilled the parts of herbivorous and carnivorous beast;, 

 and altogether the Mammalianfaunaof to-day was repre- 

 sented by analogous reptilian precursors. 



May it not have been similar in yet older periods with 

 regard to animals of the Frog class ? We have seen the 

 possibility of aerial locomotion in even the existing Rha- 

 cophorus. It is true that all existing Urodeles are fresh- 

 water forms, but it may well be that marine creatures once 

 bore the same relation to them as the great marine 

 Ganoid fish fauna bears to the few existing Ganoids* 

 which now constitute a fresh-water group. 



The great crocodile-like Labyrinthodonts must have been 

 no ignoble predecessors of the rapacious reptiles which 

 were to succeed them, and the fossil form Opludopcton 

 suggests that the existing OphioDiorpha may be the last 

 remnants of a race which preceded and represented the 

 subsequently developed serpents. 



This, however, is but a conjecture which future dis- 

 coverers will probably ere long establish or refute. 



The name Lahyiinthodonta was bestowed upon the 

 great fossil group on account of the beautiful and singu- 

 larly complex structure of -the teeth of some members of 

 the order. These teeth are conical, and exhibit slight 

 vertical grooves on their surface. A horizontal section 

 shows that these surface-grooves arc the external indi- 

 cations of deep indent itions of the substance of the tooth. 

 All these indentations converge towards the centre of 

 the tooth, but not in straight lines, each indentation 

 being elaborately inflected. Radiating from the centre 

 of the tooth are a corresponding number of processes of 

 the central pulp cavity — the radiating processes undu- 

 lating like the converging folds. 



* Existing Ganoids are tlie stni-geon, bony pike (Lepidosteus), mud-fish 

 (Lepidosiren), and others as noticed earlier. 



A similar structure of tooth is found in some Ganoid 

 fishes, and an incipient stage (as it were) of the same 

 condition existed in the Ichthyosaurus. 



We have now reviewed the closest as well as the more 

 remote allies of our Frog, and have seen how the Frog is 

 a species of a group (Anoura) which is one of three 

 existing and widely divergent orders, supplemented by 

 an extinct ordinal group of the carboniferous period — the 

 four orders (l. Anoura, 2. U rode la, 3. Ophiomorpha, and 

 4. Labyrinthodonta) being embraced in a higher unity 

 termed a " Class,'' which is the Frog's class, as " Anoura " 

 is his order. This class is with propriety spoken of as the 

 Frog's class, since the Frog is the species from which its 

 scientific derivation Batrachia is derived. This class 

 may now be considered as a whole. 



The Batrachians (of all three existing orders) are in the 

 main aquatic animals, inasmuch as the greater num- 

 ber, even when adult, frequent, at least at intervals, ponds 

 and streams, or delight in humid localities. Water also 

 is necessary for the larval stages of almost all ; and abso- 

 lutely all, at one period of lite, possess gills, while some 

 (as we have seen) retain gills during their whole existence, 

 and are permanently and constantly inhabitants of water. 

 The extinct forms (Labyrinthodonta) Avere, no doubt, 

 also aquatic, as, besides their general relation to other 

 Batrachians, traces or indications of the hard parts which 

 supported the branchiae of some Labyrinthodonts appear 

 to have been actually found. 



It is somewhat singular that in spite of this predo- 

 minating aquatic habit, all Batrachians, both living and 

 fossil, appear to inhabit, and to have inhabited, fresh 

 water only. No Batrachian of any period is yet known 

 to have been marine. This is the more remarkable since 

 the most nearly allied class, that of fishes, is much more 

 rich in salt-water than in fresh-water forms ; while even 

 existing Rcptilia have (in the true sea-snakes and in che- 

 lonians) representatives which inhabit the open ocean, 

 while in the secondary geological period marine reptiles 

 {Ichthyosauri and Plesiosauri) abounded. 



Indeed, of all classes of vertebrate animals, this aquatic 

 class [Batrachia) has the least to do with the ocean, for 

 many birds, and a still greater number of Mammals 

 [e.g. the whales and porpoises), are constant inhabitants 

 of salt water. All the adult Batrachians feed on animal 

 substances, generally small worms, insects, or slugs, and 

 animals allied to slugs. The larger Frogs and Toads will, 

 however, as has been said, devour vertebrate animals, 

 such as mice and small reptiles and birds. The existing 

 large, tailed Batrachians devour fishes. The extinct tailed 

 Batrachians, in their adult condition, were also un- 

 doubtedly animal feeders, but they may, in their young 

 state, have been vegetarians. At any rate the tadpoles of 

 the existing Urodela will eat vegetable matter, and indeed 

 probably sustain themselves mainly upon it. 



In cold latitudes the Batrachia, like the Reptilia, go 

 into the winter sleep called hibernation, as also do the 

 hedgehogs and bats amongst Mammals. 



Tire Frogs and Toads sometimes hide and shelter them- 

 selves by creeping into out-of-the-way holes and corners, 

 but more generally they (as also the Newts) bury them- 

 selves in mud at the bottom of ponds and streams. In 

 hot latitudes, some forms pass the dry season in a similar 

 state of lethargic inactivity. 



Many beasts, birds, and fishes, range in flocks. The 

 Batrachians, however, usually wander about in a solitary 

 manner, and only congregate in the breeding season. It 

 is then that their vocal powers find utterance, though only 

 in the Anourous order ; the tailed Batrachians never 

 make more than a very feeble sound. 



As regards the geographical distribution of the whole 

 class, the northern hemisphere, and especially the Ame- 

 rican portion of it, is the more richly furnished. Africa, 

 India, and Australia, are the most poorly supplied on the 

 ■whole, because, though possessing very many kinds of 



