346 



NATURAL S2STOSY. 



The bodies of the vertebrae in most of the adults are concave in front and rounded behind (pro- 

 coelous), with the exception of the eighth, or pre-sacral, which is amphiccelous, and the ninth, or sacral, 

 which has commonly one convexity in front and two behind. The vertebrae in front of the sacrum are 

 never more than nine, and the tail part is in the form of a bony style, with two rounded arches. 



The vertebrae are opisthocoelous, or hollow behind, in the genera Pipa and Bombinator ; and in 

 these, as in all other Amphibia, the bodies and inter-vertebral substances contain more or less distinct 



remains of the notochord. A sacral vertebra 

 always exists, and its transverse process, and 

 those of the vertebrae in front and behind, 

 with which it is anchylosed, are large and 

 usually expanded. 



The shoulder-girdle consists of, in the 

 Frog, for instance, the shoulder-blade (in 

 two movable pieces), the collar-bone, and the 

 coracoid bone, and all these combine to form 

 the joint cavity for the humerus. The collar- 

 bone is connected with its fellow of the other 

 side at the median line of the body, and the 

 broader and larger coracoid meets its fellow 

 also. The sternum consists of several pieces 

 which extend from the front, anterior to 

 the collar bones, to well behind the cora- 

 coid s, where it ends in a broad cartilage. 

 The front part is formed by the episternum. 

 There are no ribs. 



As the fore limbs are not of the impor- 

 tance of the hinder, the humerus is small, 

 short, thick, and has almost a globular sur- 

 face for the articulation of the bones of the 

 fore arm. These are united in one. The 

 wrist bones are six in number, and support 

 four metacarpal bones, and the index and 

 middle fingers have two phalanges each, 

 and the others three. The thumb is small 

 and rudimentary. 



The bones of the well-developed pelvis 

 present considerable differences in the various 

 genera. Thus, in the Frogs (Rana\ and 

 the Tree Frogs (Hyla), the iliac bones are very long, and are movable on the sacrum ; and 

 they are very close together below, towards the joints for the long thigh bones, so that the 

 two heads of these bones seem to be placed in contact. This peculiar arrangement influences the 

 action of the hind limbs upon the trunk in the exertion of swimming and leaping. In the 

 Pipa, or Surinam Toad, the iliac bones are very much widened at the point of junction with the 

 sacrum, to which they are fixed, and which is itself dilated. The bones of the leg (tibia and fibula} 

 are, in the Reptiles, generally distinct ; but in the Frogs and their allies they are so soldered together 

 as to form but a single articulation with the femur and tarsus, and to present the appearance of a 

 single, very much elongated, bone. The knee-joint and articulating bones are so disposed that the 

 feet have always a direction outwards. 



The united leg bones are longer than the femur, and are followed by very long astragalus and 

 calcaneum bones. Four small ankle bones exist, and the metatarsal bones and phalanges are very 

 long, as they have to support the web when it exists, and to assist in swimming and jumping. 

 The inner toe is well developed, and the fourth is the longest. 



It is remarkable that the muscles of the abdomen should be more developed in these Anoura than 



SKELETON OF A FROG, 

 (a) Sternum, &c. 



