Pahker. — On the Origin of the Sternum. 121 



border. The portion of the continuous cartilage lying above 

 (dorsad of) the articulation of the fin is tlie scapular region ; 

 that below (ventrad of) it the coracoid region. In the em- 

 bryo the shoulder-girdle consists of distinct paired cartilages, 

 which afterwards unite with one another in the middle ventral 

 line by concrescence of their coracoid portions. 



In a skeleton of Notldanus indicuH, prepared a few months 

 ago for the Otago University Museum, the middle region of 

 the shoulder-girdle (figs. 1 and 2) is produced in front into a 

 blunt process, while it is evenly curved posteriorly. Two 

 curved areas of fibrous tissue, with their convexities towards 

 the median plane, extend from the anterior to the posterioi- 

 border, touching one another in the centre, and thus bounding 

 two distinct cartilaginous areas — an anterior {pr. om. st.) of a 

 rhomboid, a posterior {i)t. om. st.) of a triangular form. The 

 two cartilages are particularly well seen when the shoulder- 

 girdle, which has been prepared by impregnation with car- 

 bolized glycerine jelly, is held up to the light. 



I think there can be no doubt that the anterior rhomboid 

 cartilage {pr. am. si.) is to be considered as a pre-omosternum, 

 the posterior triangular piece {pt. ovi. st.) as a post-omo- 

 sternum, the coracoid portion of the shoulder-girdle being re- 

 lated to the two median elements in much the same way as 

 the coracoids of Anura to the sternal cartilages compare figs. 

 2 and 3). 



It may be objected that tlie cartilages in question are un- 

 paired in Notidanus, while in Amphibia they arise from the 

 union of paired chondrites. But, in the first place, we know 

 nothing of the development of the shoulder-girdle in Notidamis, 

 and, in the second place, an actual unpaired origin would only 

 mean that the sternal elenients were detached after the union 

 of the coracoids with one another, each of them being mor- 

 phologically paired since each is derived in an equal degree 

 from the two originally separate halves of the pectoral arch. 



It will be seen that the omosternum of Notidanus is related 

 to the shoulder-girdle in much the same way as the copulas 

 (basi-hyal, basi-branchials) to the visceral arches. 



The question then arises as to whether tJiere is any genetic 

 connection between the omosternum of Notidanus and Am- 

 phibia and the costal sternum of Amniota. I am disposed to 

 think that the latter is derivable from the former, its present 

 mode of origin being a case of retarded development. 



In the early Anmiota we may suppose there to have been 

 a post-omosternum, developed as in Ampliibia, but separated 

 from the coracoids before chondrification, and joined at a late 

 jjeriod of development by the first pair of ribs. In a subse- 

 quent stage of evolution we should have the second and follow- 

 ing ribs becoming successively ujiited in the same, manner. 



