| OCR Text |
Show 68 ON CLASSIFICATION. (F· 33) E 1 amus of the lower jaw is composed of a number 1g. . ac 1 r . . . f · d art1o preces, an 'culates w1th the skull, not dnectly, but by tho Fig. 33. FJ.O ' 3..... '1'1 · ·t..,l condyle of a Crocodile's skull Yiewed from behin<l.-B.O, Basi- 0 .- 1e occtpt " . . . 1 ::o · occipital; E.O, Ex-occipital; S.O, upra-ocetptta. intervention of a bone-the os quadratu1n-with which the hyoidean apparatus .is not immediately connected (Fig. 34). Fig. 34. '.D Fig. 34.-The skull of a Lizard (Cyclodus).-D D, Dentary piece of the lower jaw; Qu, Os quadratum; Sq, quamosal. The fore-limb of Reptiles never takes the form of a wing, such as is seen in Birds; the "wing'' of the remarkable extinct flyi~g reptiles, the Pteroclactyles, being constructed on a to:ally ~tfferent principle from that of a bird. In no known reptile, agam, are the metatarsal and tarsal bones anchylo od into one bone. In all Rept-ilia the greater and lesser circulations are directlY connected together' within or in the immediate neighbourhood ' . of, the heart ; so that the aorta, which is formed by tho umon of two arches, contains a mixture of venous and arterial blood. TilE AVES. (jf) Tho blood is cold, and the majority of tho blood-corpuscles ar red, oval, and nucleated. The bronchial tubes are not connected at the surface of the lungs with terminal saccnlar dilatations, or air-vesicles. When, as is ordinarily the case., the Fmperficial layers of the epidermis of Reptiles are converted into horn, the corneous matter takes the form of broad plates, or of overlapping scales, neither plates nor scales being developed within pouches of the integument. The class of AvEs consists of animals so essentially si1uilar to l{eptiles in all the most essential features of their organization, that Birds may be said to be n1erely an extremely modified and aberrant Heptilian type. As I have already stated, they possess an amnion and a respiratory allantois, and the visceral arches never develop branchial appendages. 'l'he skull is articulated with the vertebral column by a single condyle, into which the ossified basi-occipital enters largely. Each ramus of the lower jaw, compo. ed, as in Reptiles, of a number of pieces, is connected with the skull by an os quadratum, to which the hyoidean apparatus is not suspended. In no existing bird does the tenninal division of the fore-limb possess more than two digits terminated by claws, and the uletacarpal bones are' commonly anchylosed together, so that the "manus" is of 1ittle use, save as a support for feathers. In the hind limb of all birds the tarsal and metatarsal bon s become more or less completely fixed, and the latter, anchylo"' 'd together, so as to form a single osseous mass, the "tarso-1n tatarsus." The greater and lesser circulations of birds are completely separate, and there is only one aortic arch, the right. The right ventricle has a muscular valve. The blood is hot, hotter on th average than that of any other vertebrates, and the majority of the blood-corpuscles are oval, red, and nucleated. 'l'he Lr n ·chial tubes open upon the surface of the lungs into au:- a s, which differ in number and in development in diffi rent bird . Lastly, the integument of birds is always provided with h rny appendages, which result from the conversion into horn f th cells of the outer layer of the epidermis. But the majority of |
