iii Iclithyosauria and Sauropterygia. 253 



Professor H. G. Seeley certainly has since represented the inter- 

 clavicle as in part covered by the sternum in Keirognathus cordylus* 

 but I venture to suggest that in the specimen the appearance is not 

 wholly free from ambiguity, and that its damaged condition makes 

 confirmation of so singular a deviation from the usual, relation of in- 

 terclavicle and sternum very desirable. 



It was its deep position, together with evidence suggestive of a 

 composite origin, which led me to suggest that this part or parts 

 might be omosternal, since in certain Anoura Professor W. K. 

 Parker had found an omosternum (formed by the fusion of two 

 symmetric halves, each segmented off from the epicoracoid) produced 

 above and behind the anterior border of the precoracoids, and thus 

 occupying a precisely similar position to that occupied by the bone 

 or bones in Plesiosauria. Professor H. G. Seeley's statement that I 

 regarded this bone as indivisible is a misconception, if it is intended 

 to convey that I denied it a dual origin, since in my address I dis- 

 tinctly say, " Some examples show traces of a primitive composition 

 of two similar halves. "f I have now before me a drawing of such a 

 Plesiosaurian omosternum (dated June 19, 1869) which I made of 

 a specimen in the Cambridge Museum. In its present form the 

 postero-external angles are missing it is an oblong plate, measuring 

 transversely 4' 11 ins., and 3 ins. in its antero-posterior extent along 

 the mesial line where are distinct indications of a suture showing its 

 formation by the junction of two halves. That which I regard as 

 the anterior border has the mesial notch seen in some Plesiosauria. 

 The sweep of the posterior border is broken by a projection formed by 

 the backward extension of the postero-internal angle of each of the 

 two halves. 



The form of the omosternalia evidently varied in different genera, 

 and it is the opinion of A. Leeds, Esq., who has an excellent know- 

 ledge of Enaliosaurian remains, that it varied also with age. An 

 azygos omosternal piece which I saw in his collection at Eyebury, 

 in 1883, was a moderately thin plate, having the outlines of an 

 isosceles triangle, the base of which measured 11 '6 cm., and the 

 height 11'3 cm. Its apex was rounded off, and its base slightly in- 

 curved. In the direction of a perpendicular from the apex on the 

 base, and also transversely to this direction the figure was slightly 

 curved, which gave a gentle concavity to that surface which I con- 

 jectured to be upper. 



Two omosternalia, a pair, belonging to another skeleton, had the 

 form of scalene triangles. Of the more perfect of the two the base 



* < Phil. Trans.,' B., 1888, p. 494, Fig. 2. (P. 493, line 3 from bottom, Sternum, 

 the words " so far as to underlap the posterior borders of .the coracoid," show this 

 " Eestoration" to be an inferior, or ventral, view). 



f Hulke, J. W., p. 48, Proc. Geol. Soc.,' Presidential Address, 1883. 



