76 THE STERNUM. [chap. 



the first pair of sternal ribs, and terminating anteriorly in a 

 conical rounded projection. 



The segments of the mesosternum are elongated, and 

 more or less four-sided, contracted at the middle, and 

 widening at each extremity. They ossify, according to 

 Parker, edosteally, or from without inwards, the bony 

 deposit commencing in the inner layer of the perichon- 

 drium, as in the shafts of long bones; and they remain 

 permanently distinct from each other. 



The xiphisternum is long, narrow, and flat, and .generally 

 ends in an expanded flattened cartilage. 1 



In the Pinnipedia, the presternum is produced considerably 

 in front of the attachment of the first pair of ribs. 



In the Insectivora, the sternum is variable in form, but 

 always more or less elongated and segmented. The pre- 

 sternum is always more or less expanded laterally in this 

 claviculated group of animals. It is bilobate in front id 

 the Hedgehog (Erinaceus), trilobate in the Shrews (Sorex). 

 In Rhyjichocyon it is broad in front, narrow posteriorly, 

 strongly keeled below, and with two horn-like processes 

 projecting outwards and forwards between the attachment 

 of the clavicles and the first pair of ribs. 



The mesosternum is usually narrow, as in the Carnivora, 

 but in the Hedgehog, where it consists of three segments, 

 it is broad and flat posteriorly. In this genus the xiphi- 

 sternum is rudimentary, whereas in the Shrews it is long 

 and ends in a flat expanded cartilage. 



The Mole {Talpa) and its nearest allies have a remarkably 

 developed presternum, which is longer than the whole of the 

 mesosternum (Fig. 34). It is strongly keeled below, except 

 at the front part, which is much thickened. On its superior 

 or inner surface it is grooved in the middle line. Laterally it 

 1 This is not preserved in the specimen figured. 



