MAMMALS. 207 



i. r., s. r.) I find a large intermediate rib, with a smaller supernumerary ossicle both above and 

 below it : whilst in the one in front of it (fig. 11) there is no distinct intermediate bone : here, 

 also, I note a curious variability, namely, in T. pela (fig. 4), the synovial joint (sy.) is below 

 the "costa intermedia ;" but in D. sexcinctus (fig. 10, sy.) it is above it. 1 



Ordo " RODENTIA." 



Examples. Hystrix, Helamys, Itatliyergus, Cavia, Dasyprocta, Arvicola, Arctomys, Mus, Lepus, 



Cricetus, Myoxus, Sciurus. 



The Rodents are more polymorphous than the Marsupials, but less so than the Edentata. 

 My illustrations give a great variety in this group ; and in the Muridae, Caviidae, and 

 Leporidae, the development of the parts is shown. The scapula in this tribe (Plate XIX, 

 fig. 17, and Plates XXIV XXVI, excluding fig. 16 in Plate XXV) is generally high and narrow ; 

 having the supra-spinous fossa smaller than the " infra-spinous ;" the spine moderately deve- 

 loped, and soon freeing itself from the neck to form a long acromion process. This latter part is 

 generally simple, but in some genera the metacromial process attains its greatest size (Plate 

 XXIV, figs. 1, 6, and 14; and Plate XXV, figs. 1 and 3, m. ac.). There is no scapular fenestra, 

 and the prse-scapula seldom projects much forwards ; it does so to the greatest extent in 

 Bathyerffus (Plate XIX, fig. 17, p. sc.), in Arctomys (Plate XXIV, fig. 14), and in Cricetus 

 (Plate XXV, fig. 7). The supra-scapular region has but little separateness, even in ossification 

 (see Plate XXVI, fig. 8, Mus. minutus ; Plate XXIV, fig. 14, Arctomys; and Plate XIX, 

 fig. 1 7, Bathyergus) : the coracoid (cr.) is always a small, blunt hook. Although the meso- 

 scapula of the Rodents is never fenestrate, yet the great depth of the " notch" between it and the 

 neck of the scapula shows the great tendency there is to the cleavage of this part from the rest 

 of the blade-bone, for the correlation of the meso-scapula to the thick muscular masses, fore and 

 aft of it, differentiates it as much from the rest of the scapula as the " en to-sternal" of the Bird 

 from the lateral regions, and from the same cause. The clavicle (cl.) is seldom altogether absent 

 in the Rodents, and the correlated cartilages suffer most degradation when it is largest, as in 

 the Sciuridae (Plate XXV, figs. 8 9, Myoxus avellanarius, figs. 12 1 5, Sciurus palmarum ; 

 and in the Beaver) . Answering to the anterior fork of the Shoulder-girdle moiety of the Frog, we 

 have an interrupted moniliform structure, and as many as four of these cartilaginous bead-like 

 segments appear on each side in many species. Besides these, there is very often another carti- 

 laginous rudiment on each side answering to a part of the base of the hinder fork in the Frog, for 

 instance, the epicoracoid. In the Guinea-pig (Plate XXIV, figs. 1 7), the order of the appearance 

 of these parts is very curious. Nearly ripe embryos show no clavicle, meso-scapular segment, 

 nor prae-coracoid ; their tissue is at that time amorphous ; but the "ornosternum" (figs. 1 3 

 o. st.) is apparent as a pair of small ovoidal segments of soft cartilage on each side above the free 

 end of the prae-sternum (p. st.). Below, on the front of the bony sheath of the first vertebral 



1 I must remark in the Order Edentata its extreme richness of morphological variety, the inter- 

 changeable characters throwing light upon their relationships inter se, and also below and above them 

 in the Vertebrate Sub-kingdom : the morphology of the skulls, well worked out in many stages, would 

 yield the most valuable results. 



