26 



OF THE STERNUM. 



One of tlie more remarkable parts of the comparative anatomy 

 of our Sydney specimen is the structure of the sternum. To 

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

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

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

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

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

 of sternum, must be considered theoretically as composed of two 

 bones ossified together at their symphysis. Now, on referring 

 to the Delpliinidce, which are perhaps of all Cetacea the nearest 

 to the Catodontidce, or sperm whales, we find (see Cuvier Oss. 

 Toss. pi. 2^4, fig. 21) that Dclpldnus Tiirsio, or bottle-nosed 

 dolphin, the sternum of which consists of three bones, has this 

 binary structure marked out in the anterior bone, which is dis- 

 tinguished 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 subtriangular ones joined by a cartilage in the middle ; 

 each with a wide head in front, and a deep emargination in 

 the middle. These corresponding emarginations answer to the 

 hole in the middle of the anterior sternum bone of Delpliinus 

 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 oblong open- 

 ing." Unfortunately, M. Cuvier does not seem to have ever 

 seen any part of the sternum of the Cachalot. He says, how- 

 ever, that the bottle-nosed dolphin has three bones in the 

 sternum, of which the second is simply rectangular, receiving the 

 articulation of the second pair of ribs where it joins the anterior 



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

 Balaena, 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. 



