September 1, 1891.] 



KNOWLEDGE, 



167 



Fig. 5. — Skeleton of an Ichthyosatje. 



the paddles ; this being the extreme modification which 

 hmbs have undergone tor the purposes of an aquatic life. 

 Since, as shown by the absence of gills and the indications 

 of remnants of a horny covering to the body, there is 

 abundant evidence that the Ichthyosaurs are descended 

 from land animals, they occupy a position among Reptiles 

 precisely analogous to that held among Mammals by the 

 whales of the present epoch. 



The long-necked Plesiosaurs, to which a special article 

 in Knowledge has likewise been devoted, were reptiles 

 equally weU adapted for an aquatic existence, but in 

 which the modification of the limbs into paddles had not 

 been carried to the same degree as in the Ichthyosaurs. 

 The evidence for the derivation of the Plesiosaurs from 

 terrestrial reptiles is even fuller than in the case of the 

 group last mentioned. 



Before taking leave of the reptiles we have to allude to 

 another totally different assemblage of extinct aquatic 

 forms, which were much more closely allied to the existing 

 Lizards, and many of which were of gigantic dimensions. 

 These creatures are generally known as the Mosasaurs, 

 and were first brought to notice during the last century, 

 when a huge skull was obtained from the upper Cretaceous 

 beds of Maastricht, on the Meuse ; the Latin name of the 

 group being taken from that river. 



These Mosasaurs, of which a large number of kinds are 

 now known, differ from the Ichthyosaurs and Plesiosaurs 

 in that the joints of their backbone, instead of having both 

 front and back surfaces either deeply cupped or nearly fiat, 

 had cup-and-ball articulations, the cup occupying the fi'ont 

 surface. This type of structure is common to existing 

 Crocodiles and Lizards ; but whereas in the former the 

 ribs articulate with the joints of the backbone by means of 

 long transverse processes jutting out from them, in the 

 latter the ribs articulate directly with the aforesaid joints. 

 Now the Mosasaurs have the latter mode of articulation, 

 and, since they agree with the modern Lizards in the 

 structure of their skulls, as well as in many other points 

 of their bony anatomy, there can be no hesitation in 

 regarding them as a group descended from the ancestral 

 Lizards which have taken to an aquatic mode of life. 



The Mosasaurs are confined to the Cretaceous epoch, 

 and thus lived side by side with the Ichthyosaurs for the 

 greater part of their term of existence, although they 

 attained their maximum development in the very highest 

 Cretaceous beds when the Ichthyosaurs and Plesiosaurs 

 seem to have disappeared. For a short time, then, these 

 creatures appear to have been the only gigantic marine 

 Vertebrates, filling up the gap left by the disappearance of 

 the Ichthyosaurs, which had not yet been occupied by the 

 Whales. They were of carnivorous habits, as shown by 

 their formidable teeth, and, hko all groups of Vertebrates 

 which have taken to a marine life, far exceeded in dimen- 

 sions any of their terrestrial cousins, the length of some 

 of the species being as much as forty feet. 

 I- We come now to the Birds, several groups of which are 

 exclusively composed of species speciaUy adapted for an 



aquatic life. Swimming birds, as a rule, support them- 

 selves on the surface of the water, taking occasional dives 

 of longer or shorter duration, and, therefore, have no need 



' to make any especial arrangements for breathing. They 

 swim by using their legs as oars, the feet being webbed, and 

 the toes folding up as the foot is brought forward after one 

 stroke to prepare for a second. As we shall see, however, 

 some species aid their swimming wicli their wings. All 

 birds that swim have relatively short legs, which are gene- 

 rally placed far back on the body, since this position gives 

 the maximum power in propeUing the animal through the 

 water. 



There are five chief groups of swimming birds, namely, 

 the Ducks, Geese, and Swans (Anseres) ; the Pelicans, 

 Cormorants, and Darters (Steganopodes) ; the Gulls and 

 Petrels (Gavise) ; the Divers, Auks, and Grebes (Pygopodesl ; 



I and the Penguins (Impennes). There are, however, a few 

 members of other groups, such as the Dipper among the 

 Passerines, and the Coot among the Rails, which are also 

 expert divers and swimmers. The circumstance, however, 

 that in neither of these instances is the foot fully webbed— 

 that of the Dipper being like the foot of a Thrush, and 

 that of a Coot (Knowledge, Oct., 1890, p. 236) only having 

 web-hke expansions on the sides of the toes — indicates that 

 the habits of these birds are of comparatively recent acqui- 

 sition, and have not induced any strongly-marked structural 

 peculiarities. 



The Anseres include by far the greater number of swim- 

 ming birds, and the admirable adaptation of their form and 

 structure to their mode of life is so well known as to require 

 no further mention. The Pehcans and their allies differ 

 from that group in that the web includes all the four toes 

 of the foot (Fig. C), instead of only the three h'ont ones. 

 In this group the Frigate Bird 

 has a shorter leg than any 

 bird of equal size. The Dar- 

 ters, or Snake-Birds, of which 

 there are four species found in 

 the warmer regions of the 



, globe, and of which examples 



J are generally to be seen in 



I the Zoological Society's Gar- 

 dens, are, however, those 

 members of the group most 

 interesting from our present 

 point of view. These birds 

 are found in fresh waters, and 

 swim with the whole of the 

 body submerged, so that only 



the head and upper part of the long and flexible neck are 

 exposed. In this position they look not unlike snakes 

 swimming on the water, when seen from a little distance. 

 It does not appear that any use of the wings is made in 

 swimming. The Gulls and Petrels, in which the hind toe 



. of the foot is not included in the web, have longer legs 

 than most swimming birds, and the legs themselves are 

 placed nearer the middle of the body. Since these birds 



6.— WkUbed Foot of 

 Pelican. 



