38 



CLAVICLE 



ently not due to real resemblance. What has happened in the 

 Alonotremata is, that the prescapular fossa is so enormously 

 expanded that it occupies the whole of the inner side of the 

 blade-bone, while the subscapular fossa which, so to speak, should 

 occupy that situation, has been thus pushed round to the front, 

 where it is divided from the postscapular fossa by a slight 

 ridge only. 



The clavicle is a bone which varies much in mammals. It is 

 sometimes indeed, as in the I'ngulata, entirely absent ; in other 

 forms it shows varying degrees of retrocession in importance ; it is 

 only in climbing, burrowing, digging, and flying mammals that 

 it is really well developed. 



In the higher ilannnalia the coracoid^ is present, but does 

 not reach the sternum as in the Monotremata. It is known to 



human anatomists as 

 the coracoid process 

 of the scapula. It 

 has been found, how- 

 ever, by Professor 

 Howes - and others, 

 that this process 

 really consists of 

 two separate centres 

 of ossification, form- 

 ing two separate 

 bonelets, which in 

 the adult become 

 firmly ankylosed to 

 each other and to 

 the scapula. These 

 two separate bones have been met with in the embryo of Lepns, 

 Sciurus, and the young of various other mammals belonging to very 

 diverse orders, such as Edentates and Primates. The separation even 

 occasionally persists in the adult. The cpestion is. What is the 

 relation of these bonelets to the coracoid of the Monotremata and 

 to the corresfionding regions of reptiles ? Professor Howes terms the 

 lower patch of bone the metacoracoid and the upper the epicoracoid : 



Fig. 29. — Sliouliler girdle, with upper end of sternum (inner 

 surface) of Shrew (<S"/r.i-), after Parker, x 7. t(. Acro- 

 mion ; c, coracoid ; cZ, clavicle ; ec, partially ossified 

 "epicoracoid" of Parker, or rudiment of the sternal 

 extremity of the coracoid ; 3?ia, . metacromial process ; 

 DISS, ossified " mesoscapular segment " ; ost, omosternuni ; 

 pc, rudiment of precoracoid (Parker) ; /J5, presternum ; 

 sr^, first sternal rib ; sr'^, second sternal rib. (From 

 Flower's Osteology.) 



^ To this category are perhaps to be referred cartilaginous pieces occurring in 

 the Rabbit, Mus and Sorex (see Fig. 29 above). 



^ "On the Coracoid of the Terrestrial Vertebrates," P.Z.S. 1893, p. 585. 



