{Shoulder Girdle and Clavicular Arch in Sauropterygia, 119 



" The Nature of the Shoulder Girdle and Clavicular Arch 

 in Sauropterygia." By H. G-. Seelet, F.R.S., Professor 

 of Geography, King's College, London. Received January 

 18 —Read February 18, 1892. 



I. Nomenclature of the bones. 

 II. Clavicular arch in Plesiosauridse. 



III. Clavicular arch in Elasmosauridae. 



IV. Classification. 



I. The Nomenclature of the Bones of the Shoulder Girdle. 



§ 1. In Ichthyosauria. 



The Sauropterygia and Ichthyosauria having formerly been com- 

 bined in the group termed Nexipoda or Enaliosauria, it has been 

 rather assumed than proved that the bones which form the shoulder 

 girdle in those orders are homologous. The Ichthyosaurian shoulder 

 girdle was well figured by Sir E. Home (' Phil. Trans.,' 1818, Part I) 

 and Cuvier (' Oss. Foss.,' PL 258). Figures by other authors agree 

 substantially (Huxley, 'Anatomy of Vertebrates,' p. 244) in showing 

 (1) that the coracoids meet ventrally in the median line ; (2) that 

 there is a notch on the anterior margin of the coracoid between the 

 median anterior cartilaginous border of the bone and the scapula, and 

 this notch varies in depth and width with the species ; (3) the 

 scapula is directed outward, upward, and forward ; (4) its articular 

 end has a posterior part which contributes with the coracoid to form 

 the glenoid cavity for the head of the humerus, a median part, which 

 articulates with the anterior articular edge of the coracoid, and an 

 anterior surface, which does not differ in its cartilaginous articular 

 aspect or thickness from the middle portion, but which looks inward 

 without any bony element of the shoulder girdle to articulate with it. 

 This condition has not been explained. At one time I doubted the 

 existence of such a surface in the undisturbed skeleton (" Pectoral 

 Arch, &c, of Ophthalmosaurus" ' Geol. Soc. Quart. Journ.,' December, 

 1874, p. 698), and some subsequent writers have restored the 

 shoulder girdle as though no such surface existed (J. W. Hulke, 

 "Presidential Address, Geol. Soc, 1883," p. 19, copied by R. Lydekker, 

 1 Cat. Foss. Rept. and Amph. Brit. Mus.,' Part II, 1889) ; (5) the 

 scapula carries the rod-like clavicle upon the anterior margin of the 

 bone, and from the posterior or ventral surface of the clavicles the 

 median bar of the interclavicle is prolonged backward ventrally upon 

 the coracoid bones. 



Since 1874 I have examined most of the Ichthyosaurian skeletons 

 from English and German strata without finding a specimen which 



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