290 -R. Lydeliker — Note on Ichthyosaurus tenuirostris. 



normal forms are I. communis and J. intermedius. In those species 

 the front paddles are characterized by their great width, and the 

 large number of longitudinal rows of ossicles entering into their 

 composition. In the group represented by I. tenuirostris, on the 

 other hand, the paddles are much narrower and longer, and have 

 fewer rows of ossicles. Thus, as is well shown in our figure, the 

 third digit, or the one taking origin from the intermedium of the 

 carpus, consists of only a single row of ossicles ; and there are 

 usually only four rows of these ossicles in the entire limb, against 

 seven or eight in the typical groups. Moreover, the radius — the 

 bone lying immediately below the humerus on the front side of the 

 limb — is a nearly square bone, with a notch on its front border ; 

 whereas in the typical group it is much wider and longer, and has 

 no such notch. 



All these characters indicate that the group represented by 

 I. tenuirostris is much less specialized than the typical one, and 

 thereby less widely differentiated from more ordinary reptiles. 

 Indeed, so different is the structure of the paddles in I. tenuirostris 

 and I. communis, that if we had these alone to deal with there 

 could be but little hesitation in regarding the former as the repre- 

 sentative of a distinct genus. Unfortunately (or perhaps our purely 

 geological readers would prefer to say fortunately), however, there 

 are certain intermediate forms which render it much more difficult 

 to define such genera ; so that for the present at least our species may 

 rest in the genus to which it was referred by its original describer. 



The specific characters of the present species are to be found in 

 its long and slender rostrum, narrow fluted teeth, and the presence of 

 a notch on the hinder border of the coracoid. Specimens in the 

 British Museum with the rostrum still more elongated than ordinary 

 were separated by Sir R. Owen as I. longirostris, but these proved to 

 be identical with the earlier I. latifrons of Konig. The magnificent 

 series of specimens from the Lower Lias of Barrow-on-Soar in the 

 Dublin Museum indicates, however, that these variations are not of 

 specific value, so that all these forms should apparently be referred 

 to I. tenuirostris. 



We may conclude this notice by calling attention to a point where 

 the British Museum Catalogue is in error. It appears from the 

 recent researches of Dr. E. Fraas that the type of I. aciitirostris, 

 Owen, has smooth carinated teeth, like those of I. platyodon, so 

 that this species should be transferred to the Platyodont group, 

 which it has been proposed to raise to generic rank as Temnodonto- 

 saurus. This leaves the name I. qiiadriscissus as the one best 

 applicable to the other specimens catalogued as I. aciitirostris. 

 Moreover, Dr. Fraas considers that J. zetlandicus, Seeley, is identical 

 with quadriscissiis ; and we are disposed to doubt the right of 

 separating I. longirostris, Jager (non Owen) from the same. Finally, 

 we observe with satisfaction that Dr. Fraas is disposed to consider 

 the Americein Baptanodon as inseparable from OpJithalmosavrus of the 

 English Oxford and Kimeridge Clays, of which such a fine series 

 has recently been acquired by the British Museum. 



