REPTILES. 



Its body, and the other on the tranfvcrfe proccfTts. The 

 iguana and the tiipinambis have only the upper part of the 

 fternum offified : it is broad, and receives fix ribs and tlie 

 clavicles ; the other ribs are free. The cameleon poH'efles 

 liitewifc the upper portion of the fternum ; but almoft all 

 the ribs have cartilages, which extend to the middle line, 

 and are there united to the oppofite ones. Frogs, though 

 they have no ribs, have ncverthelefs a very confpicuous 

 fternum. It forms on the anterior part a cartilaginous ap- 

 pendix, furnifhed by a di(k fituated below the larynx. It 

 next receives the clavicles, and then expanding, it terminates 

 at lall in another diik fituated under the abdomen, and 

 which afi^ords an origin for mulcles. There is a peculiar 

 bony cyft of unknown ufe on tlie internal furface of the 

 fternum, in the rana pipa. 



The falamander has ribs, fo fliort that they feem to be the 

 tranfverfe proceffes of the vertcbrce ; they have only one 

 point of articulation, upon which they have but little 

 motion. Thefc rudiments of ribs are twelve in number on 

 each fide. This reptile has, properly fpeaking, no fternum, 

 but its place is partly fnpplied by the bones of the (luiulder, 

 as we ftiall prefently fee. 



The flceleton of the clielonian reptiles exhibits to us 

 what appears on the firft view as a completely anomalous 

 organization. In the bony houfe which thefe animals 

 carry about with them, there is fuch a deviation from 

 tlie ordinary figure, connexion, and pofition of the parts, 

 compollng the Hieleton of otiier vertebral animals, tiiat 

 a hafty view would lead us to infer that the general mo- 

 del to which Nature in all her modifications of form and 

 pofition ever adheres, has been completely loft fight of, 

 and another fubftituted in its pLce. This inference would 

 be altogether incorreA : a more accurate examination 

 enables us to difcover in the external bony (liells of 

 thefe fmgular creatures, all the ofleous pieces which 

 belong to the cheft of a mammiferous animal or bird ; 

 fo that no eflential part is wanting in their thorax, and the 

 fmgularity depends merely on the more or lefs complete 

 ofEfication of the whole peftoral cafe, and the peculiar 

 forms refulting from this circumftance. Thus, inftead of 

 the anomaly, which a hafty glance leads us to anticipate, 

 we aftually difcover, on the contrary, a new proof of the 

 conftancy, with which an original model is retained 

 throughout whole claftes of animals, even under the widcft 

 differences of external form, as if Nature, having fixed on 

 one general principle of organization, would not be at the 

 trouble of inventing others, but rather cliofe, by the ftrangeft 

 modifications, to accommodate the organs to new fitua- 

 tions and forms. It is a new illuftration of that principle, 

 in conformity to which the fin of a whale contains all the 

 bones of an upper limb of a quadruped, the wing of the 

 bat the regular digital phalanges, the fin of a penguin or 

 feal the ufual bones of the extremities ot a bird or mam- 

 miferous animal, &c. 



The bones of the cheft in the tortoifes form a more or 

 lefs convex (hield-like covering, which conllitutes the upper 

 furface of the animal ; we call it the upper ftiell (carapace, 

 bouclier, &c.) ; and a nearly flat portion adapted to its 

 concavity below, conftituting the inferior furface of the 

 animal: this is the under fticll (plaftron). 



The back ftiell is formed by the expanfion of eight ribs 

 or olfeous ftaves, which arife from the joints of the vertebrae, 

 and terminate in a border that furrounds the whole fliell : 

 thefe bones are united together by real futures, fituated 

 tranfverfely. Above, and all along the middle part of the 

 ftiell, we obferve a row of little ofteons plates, almoft (quare, 

 intimately connefted together, and to the plates formed by 



Vol.. XXIX. 



the ribs, by fynarthrofis, and equal in number to the ver- 

 tebra:. Thefc plates reprefcnt the rings and fpineus pro- 

 ceffes of the vertebra;. The offeous margin is made up of 

 a great number of pieces, (eleven on each fide, and a fingle 

 one in the middle, before and behind, therefore 24 in tlie 

 whole,) loldered together, which bv their union form an 

 edge or border with three furfaccs, viz. the fuperior, which 

 belongs to the back ft>ell ; the inferior, which is joined to 

 the breaft-plate by a very thick leather-like Hi'in ; and the 

 internal, which prefents a groove for the reception of the 

 extremities of the ribs. Thefe pieces, which Geoffrey 

 compares to the fteriial or cartilaginous portion of our ribs, 

 are wanting in the loft tortoifes, (trionyx, Gcofl.) ; or at leall 

 remain coiiftantly cartilaginous or mcmbraHOus, io that the 

 middle only of the upper fliell is fuftained by an ofleous diflc. 



The turtles, and the foft tortoifes, are the only genera 

 in which the ribs, confounded in the upper fliell or carapace, 

 are neverthelefs diilinft, both by a prominence very appa- 

 rent on the iiifide of the fhell, and by a free unattached 

 portion of their ends projefting beyond the edge of the 

 fhcll. But the offeous difk, compofed of articulated pieces, 

 and already mentioned, extends round the ribs in the tur- 

 tles, receiving their ends, but is wanting in the foft tortoifes. 

 Its folidity is increafed in the former by the great plates 

 which cover it, while it remains flexible in the latter, and 

 is covered only by a fpecies of epidermis. 



The oflification of the intervals of the ribs, forming the 

 carapace, is perfetled gradually, and is not finifhed until 

 long after that of the ribs themfelves : it generally ad- 

 vances from the middle towards the edges. Thus, in a 

 young turtle the ribs will be found feparate from each other, 

 towards their external extremities, for half their length ; 

 while in an adult individual of the fame fpecies, the anterior 

 ribs are united throughout, while the intermediate ones are 

 feparated only through about one-fixth of their length. 



The carapace is always oval and pointed behind in the 

 turtles ; elliptical and gibbous in the land tortoifes ; eUip- 

 tical and flatter in the frcfli-water tortoifes. Its furface 

 is rough in the foft tortoifes ; elevated into various pro- 

 minences in the chelydes and the ferpentine, and more or 

 lefs fmooth in the others. 



The margin of the upper fliell affumes a different appear- 

 ance at its anterior part ; it is there a fquare piece of bone, 

 convex above, and concave below, which fuftains a fpine 

 for the attachment of mufcles. Its anterior edge has more 

 the form of a crefcent (lunula) ; there are alfo fome little 

 peculiar pieces above the tail. 



The breaft-plate of the tortoife is its fternum ; and when 

 deprived of the thick fliin that covers it, exhibits, in fome 

 fpecies, only one folid plate, formed of feveral pieces, 

 united by fynarthrofis : in others, this plate is perforated 

 quite through, and formed of feveral bones, fome of which 

 arc fituated in the middle line between the anterior and pof- 

 terior part, while others are placed laterally, and faftened 

 together by the help of the former, which fupport them. 

 For further information concerning the ofleous fabric of the 

 cheft in thefe reptiles, two memoirs in the 14th vol. of the 

 Annalcs du Mufeum may be confulted ; " Sur les Tortues 

 molles, par Gcoffroy St. Hilaire ;" " Sur les offemens Fof- 

 files de Tortues, par Cuvier." 



In the draco volans the ribs form the flceleton of the 

 wings, or thofe expanfions of the integuments between the 

 front and back limbs, which are fo called. The five pof- 

 terior ribs are elongated and bent backwards for that pur- 

 pofe. Here progreflive motion is performed by thefe 

 ribs ; but they are fuperadded for this purpofe, and make 

 no part of the organs of refpiration. The animal cannot 

 SD is 



