The five ordinary toes are present, the number usual in the hind foot of both Anura and Urodela.
As in the majority of Amphibians, the skull of the Frog has no basi- or supra-occipital; no basi- or ali-sphenoid; and no epi- or opisth-otics. The palatine and pterygoid bones are here membrane bones, at first lying on the surface of a cartilage rod, not as in Teleostean fishes ossifications in the cartilage. The pterygoid however invades the subjacent cartilage. The hyoid apparatus (Fig. 6) consists of a median basi-hyo-branchial plate (Bh.) suspended to the skull by a slender hyoid arch (St. h.). Behind the lower end of this arch a process represents the remains of the two first branchial arches, or cerato-branchials (Br.); another process on the posterior edge of the median plate represents the third branchial arch, or cerato-branchial (Br1); while ossifications represent the fourth arch, the so-called thyro-hyal (B. fry.). The Eustachian tube (Eu.) passes between the ear-capsule and the outer process of the pterygoid.
There is a remnant of the notochord in the middle of the centrum of each vertebra. A small tubercle, very rudimentary in the Frog, projects in some Amphibia, most of all in Urodela, from the centrum of the first vertebra, and fits into a pit in the basi-occipital condyle. It has been supposed in consequence that the first vertebra represents an axis, and that an atlas is fused with the skull, or rather has disappeared, leaving a slight trace behind. The urostyle in Bombinator igneus appears from Gotte's researches, to be derived from (1) three vertebrae, the xth, xith, xiith, and (2) a rod of cartilage which lies below the notochord. In the animal when 4/5 of adult size, the xth vertebra has a pair of nerve foramina behind it j the xith has a similar pair, not found in the Frog, and the xiith has the neural canal opening behind its arch. The notochord behind the vertebrae atrophies.
The part of the shoulder-girdle termed clavicle by Gotte, consists of a cartilage bar with a membrane bone overlying it. Bar and membrane bone constitute the praecoracoid of W. K. Parker. The bone is the clavicle, the cartilage the prae-coracoid of Gegenbaur. The bar is at first, as in the Chelonia, according to Gotte, a process of the scapula which grows ventrally, fuses medianly with the coracoid, and gives rise to a little mass of tissue which fuses with its fellow and forms the episternum as well as the connecting cartilage which unites the two halves of the shoulder-girdle ventrally. This connecting cartilage represents the posterior prolongation of the Lacertilian interclavicle. The hyposternum, according to Gotte, is not formed, like the sternum of higher Vertebrata, from the ventral ends of ribs, but by a chondrification of the membrane uniting the epicoracoids (median cartilage borders of the coracoids). In Bombinator and Urodela other structures are added, viz. in the former a pair, in the latter sometimes more, e. g. in Menopoma three pairs, of cartilage bands lying in the linea alba and intersections of the recti abdominis muscles.
Gotte compares them to the false ribs of the Crocodile and Hatteria, but Ruge (M. J. vi. 1880, p. 369) suggests that they are rudiments of the ventral ends of true ribs.
The ileum, according to Hoffmann, is an ileo-pubis. In a young Dactylethra capensis he found the symphysial portion to contain a pubic ossification fused in the adult to the ileum. In the middle line there was a projecting rod-like epipubis, a structure generally present in Urodela. In the Frog a pubic ossification appears to be absent, but Hoffmann mentions in Rana and Bufo 'a flat, thin, fairly strong tendon,' with the same attachment as the epipubis. The obturator nerve which perforates the cartilage in Urodela and marks off the pubis, passes over, i. e. outside the pubic region in the Anura.
It is not certain whether or no the cuneiform in the carpus represents the ulnare and intermedium as it does in some Urodela. The carpalia 3, 4, 5, do not always fuse in Anura. The astragalus and calcaneum are in some genera separate. The sixth toe is very commonly present. In a young R. temporaria it consists of a tarsale, and of a metatarsal with two phalanges which ultimately fuse, but remain separate in R. esculenta. The sixth and first toe bear nails in Rhinophrynus dorsalis, as they do with the addition of the second and third toes in Xenopus laevis.
Skeleton in general, see Hoffmann, Huxley, Ecker and Wiedersheim, p. 78, ante.
Skull of Anura.W. K. Parker, Ph. Tr. 166, 1876; 172, 1881.
Skull of Frog.Id. Ph. Tr. 161, 1871; cf. Parker and Bettany, The Morphology of the Skull, London, 1877. Basioccipital. Albrecht, Bull. Mus. Hist. Nat. Belg. ii. 1882-83.
Vertebral column.Gegenbaur, Untersuchungen zur Vergleich. Anat. derWirbel-saule bei Amphibien und Reptilien, Leipzig, 1862. Urostyle. Gotte, Entwickelungs-geschichte der Unke (Bombinator igneus), Leipzig, 1875, with Atlas, p. 391. Ribs. Gotte, op. cit. p. 381.
Sternum and shoulder-girdle.Gotte, A. M. A. xiv. 1877.
Pelvis.Hoffmann, Niederland. Archiv. fur Zool. iii. 1876-77.
Carpus and Tarsus.Gegenbaur, Untersuchungen zur Vergleich. Anat. der Wirbelthiere i., Leipzig, 1864.
Sixth Toe.Born, M. J. i. 1876; vi. 1880, p. 49. Structure of toes, etc. Leydig, M. J. ii. 1876.