38 



CLAVICLE 



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

 Monotremata 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 sliglit 

 ridge only. 



The clavicle is a. lione which varies much in mannuals. It is 

 sometimes indeed, as in the Ungulata, 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 develoY)ed. 



In the higher Mammalia the coracoid ^ is present, l)ut 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- 



biG. jy. — bliouliler girdle, with upper end of sternum (inner 



surface) of Shrew (,SV;/v.'-), after Parker, x 7. rt. Aero- ing tWO Separate 



raion; c, coracoid; c-/, clavicle ;<>., partially ossified bonelcts, which in 

 "epicoracoid of Parker, or rudiment of the sternal ' 



e.xtreniity of the coracoid ; 7na, metacroniial process ; the adult liecome 



■M.ss, ossified " mesoscapular segment " ; os<, omosternum ; f:„„,i^, nnV-i'l ep 1 t 



^;r, rudiment of precoracoid (Parker) ; ^js, presternum ; "11^1^7 ailivjioseu LO 



A7-1, first sternal rib ; .sr'\ second sternal rib. (From each Other and to 



Flower's Ostei'iloqi/.) ., i mi 



•^■^ ^ the scapula. Ihese 



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

 Sciurus, and the young of various other mammals l)elonging to xevy 

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

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

 relation of these bonelets to the coracoid of the Monotremata and 

 to the corresponding regions of reptiles ? Professor Howes terms the 

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



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

 the Rabbit, 3Ius and Sorcx (see Fig. 29 above). 



- "On the Coracoid of the Terrestrial Vertebrates," P.Z.S. 1893, p. DS;"). 



