MAMMALS. 219 



the Reptiles as well as in the Mammals (see Plate X, fig. 1, sc., Trachydosaurus ; and fig. 7, sc., 

 Psammosaurus). The lower part of the Shoulder-girdle is entirely absent in the Sirenia; in 

 their Sternum they are isomorphic with the Cetacea. In the adult Dugong there are five serial 

 sternal bones (see M. Fred. Cuvier, in Todd's ' Cyclopaedia of Anatomy and Physiology,' Article 

 "Cetacea," p. 569) ; in the young (three feet six inches in length, Hunterian Museum) I find the 

 Sternum in a higher morphological stage than that of the Dolphin, for the halves are completely 

 fused, and there is a free (though short) xiphoid process. Contrary to what is seen in the 

 Dolphin, the Sternum becomes wider backwards, but the prse-sternal ectosteal bone is the 

 first to appear: in the specimen just referred to it is the only centre present; it is a large, 

 oblong bone, not reaching the rounded front of the very long cervical portion of the pra3- 

 sternum : behind, it reaches the first pair of ribs. The rest of the Sternum, at this stage, is wholly 

 unossified, and has two more pairs of sternal ribs articulating with it : the xiphoid process is 

 very broad and short. The sternal ribs are stout, and of considerable length : they are in 

 this instance still soft, but ossify ultimately by endostosis ; they are segmented from the ribs, as in 

 the Cetacea. In the Manatee (Manatus americanus] the Sternum agrees with the simple Cetacean 

 type (see Plate XXIX, fig. 21, the upper view, one-third natural size, of the left half of 

 the sternum of a fresh specimen, dissected by Dr. Murie, which was five feet five inches 

 long). The whole Sternum is lyriform, emarginate in front, cuneiform behind, soft at both ends, 

 and having at this stage (not adult) only one bony shaft, which is somewhat carinate below. 

 Behind the two square, diverging praa-sternal horns (p. st.) the Sternum is suddenly narrowed; it 

 then bulges out, where it meets the only pair of ribs attached to it, namely, the second, and then 

 gently and sinuously converges its margins until it ends in the soft xiphoid wedge. The first 

 pair of ribs (v. r. 1, s. r. 1) approach the second at a right angle, but do not reach them, being 

 merely attached by ligament. A mass of fibrous tissue connects the small pisiform, hyaline 

 sternal rib with the ossified end of the vertebral. There is a hyaline mass at the end of the 

 second pair of ribs, then a fibro-cartilaginous mass (costa intermedia), and then a goodly terete 

 sternal rib, unossified, and composed of hyaline cartilage ; this is attached to the Sternum by a 

 strong fibrous ligament. The third sternal ribs only reach halfway to the Sternum (s. r. 3), the 

 rest being completed by a fibrous band, so that they are floating ribs : these are less than those 

 of the second rib, and unite with their vertebral part (v. r. 3) in a similar manner, but with less 

 evidence of an intermediate segment. Here we see that, whatever be the zoological affinities of 

 the Sirenia, their morphology in the thoracic region is very similar to what is found in the 

 Cetacea; as a correlate of their aquatic condition, they are arrested morphologically in a similar 

 manner, although not to the same degree. 



Ordo " HERBIVORA." 



A. PACHYDERMATA. 



Examples. Equus, Hippopotamus, Tapirus, Sus, Hyrax, 



B. RUJIINANTIA. 



. Examples. Bos, Tragulus. 



Prom those aquatic Herbivores, the Sirenia, the passage is easy to the great Land 

 Mammals, which are for the most part vegetable-feeders. For the skeletal regions under consi- 



