MAMMALS. 217 



Ordo "CETACEA." 



Eaumqrte. DeJjjJtimis- 



The scapula in the Cetacea (Plate XXIX, fig. 22, embryo of Delphinus ?, nine inches 



long, two diameters) is fan-shaped, being very broad in its supra-scapular region ; the anterior 

 angle is apt to be premorse, and the posterior greatly extended backwards. The posterior margin 

 is of great extent in this species ; the prae-scapular region (p. sc.) is of very small extent, and the 

 meso-scapular spine is very small, is turned forwards, and gives off very suddenly an adze-shaped 

 acromion (ac.) : this process is but little elevated above the coracoid (cr.), which has the same 

 shape, but is, in this instance, twice as large. The ossification of the body of the scapula is by 

 ectostosis, but there is a separate endosteal patch for the coracoid. In the adult Balfgna mysticetus 

 (see Memoirs on the " Cetacea," by Eschricht, Reinhardt, and Lilljeborg, Ray Soc., 1866, p. 128 1 ) 

 the scapula is very regularly fan-shaped, with three nearly equal sides ; its acromion is feeble, and 

 its coracoid feebler still, being a rounded process, one-third the size of the feeble acromion. This 

 latter process attains its highest development in Euphysetes Macleayi, Krefft (' Proc. Zool. Soc./ 

 1865, p. 712, fig. 6) : in that type the coracoid is also large, but only half as large as the pedate 

 acromion. The broadest of these flabelliform scapulae is seen in Sibbaldius antarcticus (Burmeister, 

 'Proc. Zool. Soc./ 1865, p. 714); and in this kind the supra-scapular margin is elegantly arcuate, and 

 the acromion very long. A similar supra-scapula is seen in Balamoptera patachonica (Burmeister, 

 'Proc. Zool. Soc., 1865, p. 195) ; but in this species the supra-scapular margin is sub-angular. 



The coracoid is aborted in the Cape Whales (Esch. and Reinh., op. cit., p. 128) : in the 

 genus Pscudorca (Ibid., p. 213), the scapula is very similar to that of the Dolphin ; the acromioni 

 however, is longer than the coracoid, and the posterior angle is more truncated. There is nothing 

 below the glenoid fossa belonging to the Shoulder-girdle, but the Sternum and its surroundings 

 show very interesting characters. In the embryonic Dolphin (Plate XXIX, fig. 22), the Sternum 

 has its highest Cetacean development ; but it is arrested at the same morphological stage as that of 

 certain Lizards (see Plate X, fig. 4, Trachydosaurus). The two moieties have coalesced in front, in 

 the middle, and near the end ; but there is an oval fontanelle (such as is common in the Lizards) 

 in the pra3-sternum (p. st., f.), and a large part of the meso-sternum and the hinder part of the 

 prse-sternum is occupied by the primordial fissure. The prse-sternum is very broad ; has a lunate 

 emargination in front, then a pair of rounded projections from the first ribs, then an elegant pair 

 of retral horns, and it then becomes suddenly reduced to scarcely more than half its breadth. 

 From the insertion of the second ribs the meso-sternum becomes gradually narrower, widening, 

 however, to join each pair of sternal ribs ; and then, behind, breaking out into a pair of short 

 horns, to which are articulated the fifth sternal ribs : this is precisely what takes place in Trachy- 

 dosaurus, even as to the number of the ribs. Submesially, the substance which was cleft to form 

 the floating ribs (fig. 22) left no spare tissue to be continued as xiphisternum ; but the diverging 

 ends of the meso-sternum (m. st.) are the termination of the costal key-stone. There are three pairs 

 of "pleurostea" (pi. o., 1 3), the first pair being manubrial ; these are ectosteal, but endostosis 

 is set up at once, and the two processes are nearly synchronous from the beginning (see a section 

 of the second pair of centres, fig. 23, pi. o., 2, eight diameters, and part of this section, fig. 26, 

 sixteen diameters) : it is here seen how equal in extent the two deposits are. The vertebral and 



1 Edited by W. H. Flower, Esq., F.R.S. 



28 



