MAMMALS. 197 



it is a moderately strong,/-shaped bar, and is weakest in Halmaturus (Plate XX, figs. 3 and 5, cl.), 

 and strongest in the Koala (Plate XIX, fig. 11, cl.). It is never simple, but always has a 

 " meso-scapular segment " (m. sc. s.) at its proximal end, and a " distal prse-coracoid segment " 

 (p. cr.) at its sternal end. I have never seen these ossified separately in the Marsupials, and 

 they always continue soft at the part furthest from their clavicular attachment. In most of the 

 genera the proximal end of this compound clavicle is attached to the acromion by a rather long 

 ligament ; but in Pliascolomys there is a small synovial cavity (sy.) formed at this part (see 

 Plate XIX, figs. 13, 15, 16, acr., m. sc. s.). A ligament is seen passing from the rudiment of 

 the head of the coracoid to the clavicle : this has no cartilage in it ; it represents, in an abortive 

 manner, the upper coracoid segment of the Bird : did it contain hyaline cartilage, it would form 

 a shoulder to the clavicle, as the meso-scapular segment forms a head. But there is another 

 element of the true Shoulder-girdle to be described, namely, the " omosternum," 1 and this attains 

 its highest development in Diddphys, and dies out in the Phytophagous genera. I have already 

 described the formation of the mesial parts of the Shoulder-girdle in the Batrachia (Rana, &c.) ; 

 here they crop up again, but seldom coalesce at the mid-line, as in the Frog. These cartilages 

 (Plate XIX, figs. 1, 2, and 3, o. st.) were, originally, continuous with the "distal prse-coracoid," 

 but became cut off by an obliquely transverse cleft, which becomes a small synovial cavity 

 (fig. 3, sy.) ; at the other end, the "omosternal moieties" are attached to the unossified 

 extremity of the prse-sternurn. 2 Pigs. 2 and 3 show these parts in the Opossum (two diameters) ; 

 in fig. 2, from below, and in fig. 3, in a transversely vertical section ; they are compressed 

 towards the Sternum, and depressed where they underlie the clavicle (fig. 2, o. st., cl.) ; their 

 proximal end is somewhat behind the distal (fig. 1). Seen from above (fig. 4), the overlapping 

 part of the clavicle is shown to be dilated, and capped with the unossified part of the prse- 

 coracoid (p. cr.), the broad omosternum (o. st.) passing beneath these parts in an oblique 

 manner. The " omosternum " of the Frog is the " serial homologue " of the Marsupial carti- 

 lages of the Salamandrine Amphibia ; and the Marsupial bones of the Monotremes and 

 " Marsupialia " are ossifications of cartilages which become segmented off from the forth-growing 

 (pedate) lower margin of the pubic cartilages, and are serially homologous with the cartilages 

 that become detached by a transverse cleft from the " prse-coracoid " bar the serial homologue 

 of the pubis. In the Dasyure (Plate XX, fig. 1, o. st.) the " oraosternals " agree with 

 those of the DidelpJtys ; but in the Phalanger (see Plate XIX, fig. 8, two diameters, and fig. 9, 

 three diameters) they are much smaller, and are best seen from below (fig. 9, o. st.) ; they are 



1 On the so-called " episternals" of the Mammalia, see an excellent paper by C. Gegenbauer, in 

 the ' Jenaische Zeitschrift fiir Medicin,' See., vol. i (1864), p. 175 et seq., and excellently translated 

 by my friend Mr. Power for the ' Nat. Hist. Rev.,' Oct., 1865, p. 545 567. Partly in observation, 

 and more in interpretation, I differ from M. Gegenbauer ; nevertheless, I highly value his paper which 

 came to me (as a loan from Professor Huxley) most opportunely when I was working out these parts in 

 Oviparous Vertebrata. 



1 The figure and description of these parts given by M. Gegenbauer (see 'Nat. Hist. Rev.,' 1865, 

 p. 546, fig. 1, ep. cl.) are wrong ; the author missed the very distinct fibrous tract which separates the 

 middle bar (" prse-sternum") from the recurved arms ("omosternum"). This confusion of parts of 

 the true Sternum with the elements of the out-lying Shoulder-girdle has vitiated the interpretations of 

 a large host of anatomists ; through this " fault hi the first concoction" we have got our most vicious 

 theories on the nature of the limb-arch. 



