BRITISH FOSSIL REPTILES. 105 



rior margins and the episternum, which pieces correspond with 

 the epicoracoids of the Lacertian Sauria and Ornithorhynchus. 

 The existence of these bones I have determined in some of the 

 beautifully worked out skeletons in the collection of Mr. 

 Hawkins. 



The clavicles are strong, elongated, slightly curved bones, 

 thicker in the middle than at their extremities, articulated by 

 an oblique suture to the transverse processes of the episternum 

 with their median extremities in contact, but not anchylosed 

 together as in the furculum of the Bird : in this respect, as in 

 their connection with the episternal bone, they correspond with 

 the clavicles of the Ornithorhynchus. In the entire mechanism 

 of the complex pectoral arch, indeed, the resemblance between 

 these very different animals is remarkably close, as Mr. Clift 

 first pointed out, while the difference which both these air- 

 breathing aquatic animals present in this part of their osseous 

 structure from the Cetacea is very striking. In the Cetacea, 

 for example, there is not any osseous bar interposed between 

 the two shoulder-joints, or the centres on which the fore paddle 

 worked, while similar movements of the fore paddles of the 

 Ichthyosaurus had, and in the Ornithorhynchus have their 

 momentum transferred to, and resisted by, not less than three 

 transverse bones, viz. 1st by the clavicles, 2nd by the epi- 

 sternal forks and the scapulae, and 3rdly by the coracoids and 

 scapulae. To what difference in the habits of these species had 

 these differences of structure reference ? Most assuredly it could 

 not relate exclusively to the necessity of rising to the surface to 

 respire air, as conjectured by Sir Everard Home* ; for this 

 necessity existed in all the three types of aquatic animals, and 

 much more imperatively in the Cetacea than in the Enalio- 

 sauria. In the Ornithorhynchus the anterior extremities are 

 directed outwards, as in the marine Cetacea and Knaliosauria ; 

 but they are destined in that quadruped to be applied not only to 

 displace water, but to be occasionally pushed against a more re- 

 sisting surface, as the dry land : in order therefore to enable the 

 fore limbs to react with due force upon the body, a strong appa- 

 ratus of bone is introduced between the two shoulder-joints, 

 whereby these parts are prevented from yielding inwards and com- 

 pressing the soft muscular masses. But in the Cetacea, which 

 were never intended to quit the deep, such an appai'atus of bone, 

 as it would have added unnecessarily to their weight, has been 

 excluded from the mechanism of their anterior extremities : and 

 hence it is that, when they have the misfortune to be stranded, 

 they are unable to regain their native element. The instrument 



* Phi!. Trans., 1818. 



