208 



KNOWLEDGE 



[NOVEMBEB 2, 1891. 



In regard to their geological distribution, Crocodilians 

 more or less closely allied to the existing Gharials, Croco- 

 diles, and Alligators, are found throughout the rocks of the 

 Tertiary period as far down as the London clay. Some of 

 these extinct species were, however, of gigantic dimensions ; 

 one from the Siwalik Hills of India, which was allied to 

 the Gharial, attaining a length of between 50 and 60 feet, 

 and tlms presenting a great contrast to living Crocodiles, 

 which rarely exceed a length of some 22 or 23 feet. 

 Another species found in the Tertiary clays of Hampshire 

 presents characters intermediate between Crocodiles and 

 Alligators, the fourth lower tooth being usually received 

 into a pit in the skull, but the under surface of the body 

 ha\ang a complete bony armour like that of the Caimans. 

 This genus [Dijilini/iiorln)!) differs from both Crocodiles 

 and Alligators in that both the third and fourth lower 

 teeth are larger than the adjacent ones, so that the animal 

 had two powerful tusks in the sides of the lower jaw. 



A few Crocodilians, more or less closely allied to existing 

 types, also occur in the Cretaceous rocks, but when we 

 reach the Wealden and .Jurassic strata nearly all the forms 

 difi'er very markedly h-oni modern times, and sliow a lower 

 stage of development. Before, however, we are in a 

 position to understand how these early Crocodilians differ 

 from their living cousins, we must enter a little more fully 

 into the anatomy of the latter. Turning once more to 

 Fig. 2, we see that the passage leading to the internal 

 nostrils is formed by four pairs of bones, on the fourth of 

 which the letter A' is placed. Again, in all Uving Croco- 

 dilians the vertebra articulate with one another by a ball- 

 and-socket joint, of which the socket is situated at the 

 front of each vertebra ; this mode of articulation being 

 the best adapted to give fi'ee motion of one vertebra upon 

 the other. The third pomt we have to notice relates to the 

 bony armour of liWng Crocodilians, in all of which the 

 pitted scutes forming the shield on the back are ridged, 

 and arranged in from four to eight longitudinal rows. 

 Moreover, m the Caimans and the extinct Diptoci/noflon, the 

 shield on the under surface of the body forms a single 

 mass, made up of more than eight longitudinal rows of 

 scutes, each of which, as already mentioned, consists of 

 two separate pieces united together by suture. 



If we now contrast these features with those obtaining in 

 the Jurassic Crocodilians, we shall find very considerable 

 differences. Thus in the skull of those reptiles the fourth pair 

 of bones on the palate did not meet in the middle line below 

 the passage to the nostrils, so that the internal nostrils were 

 placed immediately behind, or sometimes partly between, 

 the bones lying between /'/' in Fig. 2, and were thus much 

 forwarder than in modern Crocodilians. Then, again, the 

 vertebra were slightly cupped at both ends, thus admitting 

 of much less motion between one another. The armour 

 on the back of the body is of a simpler type, consisting 

 only of two longitudinal rows of scutes, which lack the 

 longitudinal ridges so characteristic of those of the existing 

 forms. On the other hand, the armour on the under surface 

 was nearly always present and more developed, frequently 

 consisting of two distinct portions, in the foremost of which 

 the scutes (which consisted of a single piece) overlapped Uke 

 slates, while in the hinder part they were joined by their 

 edges to form a solid pavement of bone. 



Like their modern cousins, the Secondary Crocodilians 

 included both long-snouted (Fig. 3) and short-snouted types, 

 the former being the more common and especially abundant 

 in the Lias, where their remains occur in company with 

 those of Fish-Lizards and Plesiosam-s. In many of these 

 forms the pit (T) in the temporal region of the skull was 

 larger than the socket of the eye, whereas in recent Croco- 

 diles it is much smaller (Fig. 2), and in the Alligators may i 



even disappear. A large number of these Jurassic forms 

 were of marine habits, and a few of them attained enormous 



Kir,. :{. -Side view of tlje skuU of an extinct Crocodilian of the 

 l.i;i<; one-fourtli tlie natui"!!! size, [jetter?* as in Fig. 1, 



dimensions, the length of the skull of one species falling not 

 much short of five feet. A few species are further peeuUar 

 in having altogether discarded their bony armour on both 

 surfaces of the body. 



Looking at Crocodilians as a whole, it is perfectly e\ident 

 that they have advanced in complexity of organization with 

 the advance of time, the backwardly placed internal nostrils 

 and ball-and-socket vertebrfe of the modern types being 

 clearly an advance on the Jurassic forms. As, however, is 

 the case in many similar instances, the gradual backward 

 shifting of the internal nostrils presents a problem difficult 

 to understand, as it is hard to conceive what advantage 

 the species in which these nostrils were situated in the 

 middle of the palate had gained over reptiles in which 

 they were placed near the muzzle, the completely backward 

 position being apparently essential in order that the mouth 

 might be kept open under water. 



With regard to the general disappearance of the inferior 

 body-armour and the invariably increased development of 

 that on the back of the recent forms, it may be suggested 

 that, as most of the Jurassic Crocodilians were of marine 

 habits, and probably swam far out to sea, it would have 

 been highly advantageous for them to have the lower sur- 

 face of the body protected from attacks from below by 

 sharks and other creatures. On the other hand, since 

 modern Crocodiles and Alligators spend a considerable 

 portion of their time on the banks of rivers, and when in 

 the water are in the habit of reposing or crawling on the 

 bottom, it is obvious that the back is the portion which 

 requires especial protection. An explanation of the exis- 

 tence of an armour on the lower surface of the body in the 

 Caimans is less easy to give, although it may be merely an 

 instance of the retention of an ancestral character. 



We may conclude this account by referring to some 

 interesting observations recently made by Dr. Voeltzkow 

 on the eggs and embryos of the Crocodile of the Nile. 

 It appears that in Madagascar the egg-laying lasts from 

 the end of August to the end of September, the number of 

 eggs in a nest varying from twenty to thirty. The nest is 

 dug about two feet deep in the dry white sand ; the bases 

 of its walls are gouged out, and into the lateral excavations 

 thus formed the eggs roU from the slightly raised centre 

 of the floor of the nest. Externally the nest is not dis- 

 cernible, but the parent sleeps upon it. The eggs differ 

 greatly in form ; the shell is white, thick, firm, and either 

 rough or smooth, the double sheU-membrane being so 

 strong that the egg keeps its form after the shell has been 

 removed. When newly laid the eggs are very sensitive, 

 and are readily killed by damp or by heat, but the older 

 eggs are hardy. When the young embryos are about to 

 be hatched, they utter distinct notes, which the mother 

 hears, even through two feet of sand, and proceeds to dig 

 open the nest. Before hatching the embryo turns, and in 

 so doing partially tears the foetal membranes. With the 

 tip of its snout turned to one end of the egg, the young 

 animal bores through the shell with a double-pointed tooth 

 comparable to that which young birds possess. This tooth 

 appears very early — by the time the embryo is sis weeks 



