564 TKE NATURAL UISTORT REVIEW. 



the episternals present themselves under three different forms in 

 Mammals. 



The first, which may be considered as the most complete 

 form (because presenting but slight modification of the type ob- 

 served in the lower Yertebrata) is that in which it consists of a 

 median piece connected with the sternum, and carrying two lateral 

 transverse portions. The two other forms are incomplete, modified, 

 and in some respects degenerated. The former more complete ar- 

 rangement serving as the type, is characteristic of the Lizards, but 

 so far as I am aware does not occur in lower groups. In the Mono- 

 tremes and Opossums (and whether also in the remaining Marsu- 

 pials further investigation must disclose) the typical form is still 

 preserved ; though in the Opossums the whole structure remains 

 persistently cartilaginous. It also still appears in certain Eodentia, 

 as in Cavia and Codlogenys^ though the lateral pieces are now separate 

 and distinct from the median portion. 



The second form is characterized by the presence of the median 

 piece alone, and is seen in the I'rog, in the Crocodile, and amongst 

 Mammals in the Pinnipedia. It thus occurs even in animals desti- 

 tute of clavicles. 



The third form, lastly, is characterized by the absence of the 

 median piece, whilst the two lateral portions are still visible. The 

 whole structure seems here to be dependent upon the presence of a 

 clavicle, and from its first appearance, with few modifications, it con- 

 stitutes a connecting medium between the clavicle and the sternum. 

 This connecting medium may either be represented by an ossifying 

 piece of cartilage, as in Mus and ^ore.v, or by a bone which is con- 

 nected to the clavicle by a ligament, as in the Armadilloes, or by a 

 fragment of fine cartilage, or by fibrous tissue alone. The cartila- 

 ginous piece representing the episternal, or more correctly speaking 

 the symmetrical lateral pieces of the episternum, is either included 

 in a longer fibrous mass as in Lepus, or is continuous mth the 

 clavicle as in Erinaceus, or is separated from the clavicle by the in- 

 tervention of a joint, as in Cricetus ; in both the latter instances 

 being but loosely connected with the sternum : or again, it may be 

 directly continuous both with the clavicle and the sternum, as in 

 Talpa ; or it may be separated alike from the clavicle and from the 

 sternum by an articulating cavity, as in Monkeys and in Man. If 

 only ligamentous tissue be present, it proceeds either from the w^hole 

 extent of the surface of the sternal extremity of the clavicle, some- 



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