ANNIVERSARY ADDRESS OF THE PRESIDENT. 45 



Are there discernible in the members of these two orders such 

 structural resemblances as command our assent to their common 

 ancestry ? 



Disregarding Placodas, as too imperfectly known, and limiting the 

 comparison to the elements of their shoulder-girdle and limbs, or, in 

 Prof. Owen's words, to their " sterno-coraco-scapular frames " and 

 limbs, there are present, in all the genera of each order in which 

 these parts are known, two pairs of bones the essential nature and 

 identity of which are undoubted, the coracoids and scapulae. 



Fig. 1. — Shoulder-girdle of Ichthyosaurus, ventral view. 



ci, the ciavicie; ic, tiie interelavicle ; sc, tiie scapula; cor, the coracoid ; 

 gl, the glenoid fossa. 



In Ichthyosaurus (fig. 1) the coracoids are relatively broad, and they 

 are deeply notched in front and behind the glenoid fossa. In the 

 typical Plesiosauri (as P. dolichodeirus) (fig. 2) the coracoids are long 

 and narrow. In Cope's subgenus Elasmosaurus, as also in P. Manseli, 

 Hulke, and Colyrnbosaurus, Seeley, which are evidently closely related 

 to it, the bony coracoid is broad, but of relatively small antero-posterior 

 extent, and it sends backwards from the postero-external angle a 

 wing-like process or cartilage leaving a vacuity perhaps subtended 

 by membrane. 



In Nothosaurus a deep narrow notch separates a long square pro- 

 cess in front of and internal to the glenoid part of the bone, which 

 a constriction divides from a large rhomboidal expansion that meets 

 the other coracoid in the middle line. Yet under these diversities of 

 form, the true homology of the coracoid is too plain to be missed ; and 

 this is equally true of the scapula, to which I shall presently return. 

 Ichthyosaurus, our sole representative of the order Ichthyopterygia, 

 has in its shoulder-girdle a third pair of bones, by general consent 

 clavicles. Are these bones present in the Plesiosaurian shoulder-girdle 

 and in that of the other genera of Sauropterygia ? Some anatomists 

 answer this affirmatively, finding clavicles in Plesiosaurus (fig. 2) in 

 that piece which extends forwards and inwards from the glenoid part 

 of the scapula to the median azygos bone reputed episternum or inter- 

 clavicle, and they consider that in this genus the clavicle is confluent 



