26 



OF THE STERNUM. 



One of the more remarkable parts of the comparative ana- 

 tomy of our Sydney spechnen is the structure of the sternum. 

 To understand this structure, it may be useful to bear in mind a 

 remark of Geoffrey de St. Hilaire,that the bones of symmetrical 

 animals are always in pairs, one ranged on each side of a 

 theoretical spinal axis or medial line ; so that a central, or what 

 appears in nature to be an odd bone, such as a vertebra or a 

 bone of sternum, must be considered theoretically as com- 

 posed of two bones ossified together at their symphysis. 

 Now, on referring to the Delphinidce, which are perhaps of 

 all Cetacea the nearest to the CatGdontidce, or sperm whales, we 

 find (see Cuvier Oss. Foss. pi. 244, fig. 21) that Belphinus 

 Tursio, or bottle -nosed dolphin, the sternum of which con- 

 sists of three bones, has this binary structure marked out in 

 the anterior bone, which is distinguished by a hole in the 

 centre of the ossified symphysis,* and in the third bone by the 

 trace of a central suture. In our Sydney sperm whale, the 

 anterior bone must be described as two distinct sub-triangular 

 ones joined by a cartilage in the middle ; each with a wide 

 head in front, and a deep emargination in the middle. 

 These corresponding emargination s answer to the hole in the 

 middle of the anterior sternum bone of Belphinus Tursio, 

 which, as before said, has the two bones consolidated into one. 

 So also Beale describes the anterior piece of the sternum in 

 his sperm whale to be " perforated in the middle by an ob- 

 long opening." Unfortunately, M. Cuvier does not seem to 

 have ever seen any part of the sternum of the Cachalot. 

 He says, however, that the bottle-nosed dolphin has three 

 bones in the sternum, of which the second is simply rectan- 

 gular, receiving the articulation of the second pair of ribs 



* It woidd appear according to Cuvier, that the true whales or genus 

 BalcBJia, have not got this perforation in the solid anterior piece of their 

 sternum; so that we have here another proof of sperm whales being nearer to 

 dolphins than to true whales in their structure. 



