The clavicles, when united, as usual, form the furcula; mostly the distal median portion is drawn out into a hypocleidium of various shape.
In birds of various groups the clavicles are more or less degenerated, the reduction beginning at the distal end.
In none of the existing, and in but few of the extinct types, are collar-bones, or clavicles, developed; and the scaphoid and lunar bones of the carpus are separate.
These rodents are characterized by the imperfectly rooted cheek-teeth, imperfect clavicles or collar-bones, cleft upper lip, rudimentary first front-toes, smooth soles, six teats and many cranial characters.
- Amphicoelous; parietals separate; clavicles dilated and with a perforation near the ventral end.
- Amphicoelous; parietals separate; but the nasal bones are fused together, and the clavicles are not dilated.
- Procoelous; parietals united; eyelids functional; clavicles expanded as in the true geckos which they resemble in other respects.
Procoelous vertebrae; ventral portions of the clavicles not dilated; parietal bones fused into one.
Rodents may be characterized as terrestrial, or in some cases arboreal or aquatic, placental mammals of small or medium size, with a milk and a permanent series of teeth, plantigrade or partially plantigrade, and generally five-toed, clawed (rarely nailed or semi hoofed) feet, clavicles or collar-bones (occasionally imperfect or rudimentary), no canine teeth, and a single pair of lower incisors, opposed by only one similar and functional pair in the upper jaw.
The scapula is usually narrow, with a long acromion; the clavicles may be altogether absent or imperfect, as in porcupines, cavies and hares, but in most species are well developed.
Except in Lophiomys, the clavicles are complete.
The hollow tympanic bullae, they have the clavicles imperfect, the first front toe opposable to the rest, the temporal region of the skull roofed with bone, and the crowns of the molars with cusps arranged in rows but eventually covered by a layer of enamel.
In the Bathyergoidea the scaphoid and lunar of the carpus are separate, the tibia and fibula united and the clavicles normal.
The tibia and fibula are separate, but the scaphoid and lunar are united, and the clavicles are generally incomplete.
The upper lip is cleft, the jugal lacks an inferior angle, the fore part of the skull is short and broad; the cheek-teeth are partially rooted, with external and internal enamel-folds, the soles of the feet are smooth, there are six pairs of teats, the clavicles are imperfect and the tail is not prehensile.
All the New World porcupines, representing the family Erethizontidae (or Coendidae) are arboreal in their habits, and have the upper lip undivided, the cheek-teeth rooted, the clavicles complete, the soles of the feet tuberculated and three pairs of teats.
Many of them, like ungulates, are specialized for swift running, and have unusually long limbs, with ridges developed on the articular surfaces of the lower bones; the clavicles are more or less reduced; the thorax is more compressed than usual, with a narrower breast-bone; and there is a marked tendency to the reduction or loss of the lateral toes, more especially in the hind limb.
Fore-feet with four digits, hind-feet with three; clavicles imperfect; molars divided by enamel-folds into transverse lobes; milk-teeth shed before birth.
They are small rodents with complete clavicles, foreand hind-limbs of nearly equal length, no external tails and short ears.
A parietal foramen; scales or bony scutes frequently present, especially on the ventral region, which is further protected by three large bony plates - interclavicle and clavicles, the latter in addition to cleithra.
The clavicles rest on a large discoidal, rhomb o i d a 1, or T-shaped median bone, which clearly corresponds to the interclavicle of reptiles.
It is now possible to define the suborder Hyracoidea as including ungulates with a centrale in the carpus, plantigrade feet, in which the first and fifth toes are reduced in greater or less degree, and clavicles and a foramen in the lower end of the humerus are absent.
Society, 18 74, p. 594) to be "lost," whereas the clavicles, which in most birds unite to form that bone, are present, though they do not meet, while in like manner the bird has been declared (op. cit., 1867, p. 624, note) to furnish among the Carinatae " the only apparent exception to the presence of a keel" to the sternum.
From the picas the hares and rabbits (Leporidae) are distinguished by the imperfect clavicles, the more or less elongated hind-limbs, short recurved tail (absent in one case) and generally long ears.