MAMMALS. 209 



an immature specimen of H. alojjhiis (Hunt. Mus., No. 2113) the meso-scapular segment (fig. 14, 

 m. sc. s.) lias become articular cartilage to the small clavicle (cl.) ; the rounded prse-coracoid (p. cr.) 

 ossifies independently ; and each omosternal (o. st.) is a long, rib-like ray, with an osseous centre 

 for its pith. In Bathycrgus maritiinus (Hunt. Mus., Nos. 2246, 2247) the meso-scapular segment 

 (Plate XIX, figs. 17,1 8, m. sc. s.) is a considerable, carpiform bone, quite independent of the clavicle 

 (cl.), as also is the broad, rounded prse-coracoid (p. cr.). In this type the omosternum has been 

 reduced to a ligamentous condition ; as it is also in the Beaver, and in the next example. In 

 Helamys capensis (Plate XIX, fig. 19) the meso-scapular segment is used up by the clavicle (cl.), 

 but the pra>coracoid (p. cr.) is a solid, self-ossified mass. In the tame Rabbit, the clavicle and 

 the cartilages cannot be found at birth (see Plate XXV, figs. 1 and 2) ; in a specimen (wild) 

 seven or eight months old, all these parts can be seen (figs. 3 and 4 ; natural size and two dia- 

 meters) ; the meso-scapular segment (m. sc. s.) is semi-oval, and quite free of the feeble clavicle 

 (cl.) ; the prse-coracoid (p. cr.) is small, and attached to the distal end of the clavicle ; and cleanly 

 cleft from this is the long, nail-like omosternum (o. st.) : the whole chain of structures only takes 

 up half the distance between the acromion and the prae-sternum (ac. p. st.). In Arctomys ludovi- 

 cianus (Plate XXIV, figs. 14 16 ; natural size and three diameters), the meso-scapular segment, 

 in the adult, merely forms a film of cartilage over the upper end of the clavicle. The other end 

 is made clavatc by the addition of the prse-coracoid (p. cr.), which has, however, its own osseous 

 centre. The seed-like omosternals have their own internal centre (o. st.), and at each end of 

 these there is a small joint-cavity (fig. 16.) In Cricelus vulgaris (Plate XXV, figs. 6 and 7 ; one- 

 and-a-half and three diameters), the meso-scapular and prse-coracoid segments (m. sc. s., p. cr.) 

 form a cap to the ends of the clavicle (cl.) ; the omosternal segment (o. st.) continues soft, at least 

 for a long period. The clavicle uses the meso-scapular segment and the prse-coracoid as articular 

 cartilage in Myoxus (Plate XXV, figs. 8 and 9) ; and also in Soiurus palmarum (figs. 13 and 15). 

 In these Sciurine types the omosternals (o. st.) are mere meniscoid fibro-cartilages, of a triangular 

 form, and imbedded in the sterno-clavicular ligaments (see figs. 9 and 13, o. st.). 



The Sternum in the Rodentia agrees very much with the same part in the Marsupials ; the 

 segmentation is, however, less complete ; but the principal bones are ectosteal. Except in the 

 lowest types (e.g. Cavia, Lepus), the manubrium is broad; and in many instances the cervical 

 part of this division of the sternum is but little produced. 1 



The transverse cleavage of the general sternal bar takes place to a less extent than in the 

 Marsupials ; and when it is completest, for instance, in Arctomys (Plate XXIV, fig. 14) and Mus 

 (Plate XXVI, figs. 5, 8, and 12) there is no joint-cavity, that I can discover. In Helamys (Plate 

 XIX, fig. 19) the cartilaginous mass between the shafts is only notched below, and ultimately it 

 becomes converted into an epiphysis belonging to the shaft fore and aft of it. Such an azygous 

 epiphysis can be seen in Mus decumanus (Plate XXVI, fig. 13, ep.), even when the segmentation 

 is more perfect, a small grain of cartilage being left in the fissure. In the new-born Rabbit, the 

 manubrium has become nearly cut off (Plate XXV, figs. 1, 2, p. st.) ; but this fills in again. After- 

 wards an azygous epiphysis appears in the interspaces of the meso-sternal region (Plate XXV, 



1 It is this part of the Sternum which M. Gegenbauer supposes to belong to the Shoulder-girdle, 

 as. the " median episternum," in Ccelogemjs ; it is described as being segmented off by fibrous tissue 

 ('Nat. Hist. Rev.,' 1865, p. 548, fig. 2 m.). I have already shown that this part is separated from the 

 costal half of the prce-sternum by a synovial joint in Pholidotm Dalmannii (PL XXII, fig. 1, pr. o.) ; 

 it is merely the coalesced tips of the front sternal horns. 

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