Page:Encyclopædia Britannica, Ninth Edition, v. 20.djvu/473

Rh SKELETON.] KEPTILES 455 lower arm bones, the ulna is stronger than the radius, and its olecranon forms a short projection. The carpus consists of nine bones, one at the end of each loAver limb, one at the base of each metacarpal, one the os centrale between these five and the two at the end of the midlimb bones, and one the pisiforme outside these latter two. There are five metacarpals and five digits, the latter composed of pha- langes, which for the most part number two for the inner- most digit or pollex, three for the next or index, four for the third digit, five for the fourth (generally the longest) digit, and three for the outermost or ulnar digit. In the Chamaileons the five proximal carpals coalesce with the metacarpals, and the phalanges are 2, 3, 4, 4, and 3 respectively; the digits, moreover, are very exceptionally disposed, the three inner or radial digits being opposed to the two outer or ulnar ones. The digits may be but four in number, as in Saurophis, GymnopJithalmus, Tetradactyhis, and others ; or but three, as in Chalcis, Seps, and others ; or but two, as in Ileteromeles, Chelomeles, and others; or, finally, but one, as in Rhodona, &c., as we have already seen. The limb may be entirely wanting, as in Pygopus, Delma, Lialis, Anguis, Acontias, and others. In the Oplddia the pectoral limb is entirely absent, not even a rudiment of it being found. In Hatteria the limb is Lacertilian in form but has ten carpals. The Crocodilia have the long limb bones well developed. The carpus has two large long ossicles articulated with the radius and ulna respectively, and a pisiforme; and an oblong bone and disk of cartilage lie between the large carpals and the five metacarpals. The third digit is the longest, and the number of phalanges of the digit from within out- wards (as before) is 2, 3, 4, 4, 3. In the Chelonia the humerus may be nearly straight, as in Chelone, or very much curved, as in Testudo. The radius and ulna, which, though susceptible of very little motion, are generally distinct, may become anchylosed to- gether towards their distal ends, as in Chelone. The carpus has generally nine ossicles. The pisiforme may be very large, as in Chelone. There are always five digits, which both their metacarpals and phalanges may be very long, as in Chelone, or exceedingly short, as in Testudo. The number of the phalanges may be but two to each digit, as in Testudo, but generally it is 2, 3, 3, 3, 2, from pollex to the ulnar digit. Fdc The Pelvic Limb. The skeleton of this limb, like that 1'k of the pectoral one, is in its main features normally con- ditioned as is that of Mammals, save as to the mobility of the tarsus, as will be shortly explained. It may, however, be simplified in two ways like the pectoral limb namely, by the great similarity and want of differentiation of its numerous parts, as in Ichthyosaurus, or by excessive redac- tion in the number of its parts, as in certain Ophidians, and in Saurians such as Pseudopus and Lialis. Its resemblance to the pectoral limb, which, as we have seen, is so exceedingly close in Ichthyosaurus and Plesio- saurus, may be, in other conditions, hardly less complete in some Chelonians, such, e.g., as Chelydra. When all four limbs are fairly developed, the pelvic ones are in no existing forms so inferior in size to the pelvic ones as in the extinct Ornithosauria, or so superior in size to the latter as in some of the extinct Dinosauria, as, e.g., in Compsognathus. Amongst existing Reptiles the Lacertilians present us with very varied conditions. Generally there is an elon- gated femur furnished proximally with a trochanter on its inner side. Of the lower limb bones the tibia is larger than the fibula. A knee-pan bone, or patella, is placed in front of the junction of the upper and lower limb bones. The tarsus consists of a proximal segment made up of two large ossicles more or less firmly and immovably united by fibrous tissue to the tibia and fibula and of a distal segment, which consists of a cuboid bone and which is capable of motion upon the proximal segment, while it is firmly connected with the five metatarsals. Thus the flexion of the foot upon the leg takes place, not as in Mammals between the whole tarsus and the leg-bone, but between the distal and proximal segments of the tarsus itself. The outermost or peroneal metatarsal is more or less bent toward its proximal end. The fourth digit is the longest, and the number of the phalanges of the digits, from within outwards, is 2, 3, 4, 5, 4. In the Chamseleons the distal tarsals coalesce with the ineta- tarsals, and two inner or tibial digits are opposed to the three outer or peroneal ones. The foot may nave but four digits, as in Campsodactylus, Tetradactylus, and others ; or but three digits, as in Se]is t Heteromeles, and others ; or only two, as in Chelomeles, Rhodona, and others ; or but a single one, as in Chalcis. There may be but a very small and slender femur, to which a still smaller tibia unites a rudiment of a tarsal ossicle at its distal end, as in Lialis, or but a rudiment of a femur and tibia only, as in Pseudo- pus; or the limb may be entirely wanting, as in Anguis, Acontias, Chirotes, Amphisbsena, and Lepidosternon. In the Ophidia the limb is generally wanting, and there is never any rudiment of the tarsus or digits, but there may be a rudiment of a femur, as in Stenostoma, or of a tibia as well as a femur, the tibia ending in a hooked pro- cess, as in Cylindrophis, Ilysia, and Boa. In Hatteria the limb is as it normally is in Lacertilians. In the Crocodilia there is a well-developed femur, and also a tibia and a fibula. The tarsus has two proximal bones (less closely united than in Lizards), whereof that adjoining the fibula has a large calcaneal process. The distal seg- ment of the tarsus consists of two rounded ossicles on the peroneal side and a thin plate of cartilage on the tibial side. There are four long tibial metatarsals supporting digits and a rudimentary fifth one to which no rudiment of a digit is attached. Of the four digits the third is generally the longest, and, like the fourth digit, has four phalanges. The second digit has three, and the inner- most, or hallux, but two. In Chelonians the femur has a strong, rounded articular head, which forms a marked angle with the body of the bone. Sometimes, as in Emys and Trionyx, there are two troch- anters separated by a groove. The lower limb bones are always distinct. They are longest in the Land Tortoises, and shortest in the Marine Turtles. There is no patella. The tarsal bones vary by coalescence from eight to six in number. They are exceptionally flattened in Chelone. In Testudo the metatarsals are short, but are longer than are the metacarpals, whereas the reverse is the case in Chelone. There are always five digits, except in Testudo, where the fifth is only represented by a rudimentary metatarsal. The metatarsals are shortest, in Testudo and its allies, though they are not so short as are the digits of the hand. They are longest in the aquatic forms, though they are not so long in Chelone as are the digits of its hand. The number of phalanges, counting from within outwards, may be 2, 2, 2, 2, as in Testudo, or 2, 3 3 3, 2, as in Trionyx, or 2, 3, 3, 3, 3, as in CJielone. Myology. The muscles of four-footed Reptiles other than the Muscles. Chelonians are arranged in a general way on the same fundamental plan as in the Mammals, though the deter- mination of the true homology of many of them is more or less difficult if not impossible now to determine. The muscular masses are thus arranged in a similar longitudinal manner on the dorsum of the trunk, and expand into large