408 ON THE COLUMELLA OF ICHTHYOSAURUS. [Juiie 29, 



displaying an uncrushed inner view : the bone, however, though well 

 shown in Mr. O'Neill's drawing, is not specially marked, and it is 

 therefore advisable to append a separate sketch, such as is given in 

 the woodcut, fig. 3 (p. 407). The general outline is similar to that 

 of the examples already described, but the additional characters of 

 the inner aspect are well worthy of note. In the constricted portion 

 of the bone, the shaft is compressed to form a sharp ridge, which 

 terminates in an abrupt prominence at the point where the lower 

 expansion commences, and beneath this the broad surface is divided 

 into two distinct, apparently articular, facettes. The upper and 

 hinder division (a) is slightly hollowed and somewhat triangular in 

 shape ; while the lower facette (6) is more elongated, and is separated 

 from the first in its anterior portion by being more deeply impressed 

 in the bone. 



Among the crushed cranial bones, immediately behind the sclerotic 

 plates, in another specimen of Ichthyosaurus in the National Collec- 

 tion the Columella is also distinctly visible; but this does not supply 

 any additional facts of imj)ortance. 



On comparing the bone under consideration with its homologue 

 among recent Reptiles, none is found to exhibit a more striking 

 similarity than that of Hatteria (fig. 4, p. 407). As Dr. Giinther has 

 pointed out ^ the columella in this genus is particularly remarkable 

 for the great expansion of its extremities ; and it is also peculiar from 

 the fact that the lower end articulates not only with the pterygoid, 

 but also with an inward extension of the quadrate. Moreover, so 

 far as can be ascertained from a complete skull, the columella 

 appears to show some signs of contracting this articulation by an 

 overlapping of the two bones in a vertical plane ; and the upper end 

 is connected with cartilage, and not directly in contact with the 

 parietal above. 



Unfortunately at present it is only possible to compare the/or»i 

 of the element in each of these types. In none of the fossil Ichthyo- 

 saurs 1 have examined are the precise relations of the bone very 

 distinct. As already stated, however, the first fossil is remarkably 

 suggestive of a direct articulation of the upper end of the columella with 

 a downward process of the parietal ; and the originals of figs. 2 and 3 

 exhibit so close a resemblance to the corresponding parts in Hatteria, 

 that there is also strong evidence of the lower articulation being 

 double. But it ought to be remarked that in Ichthyosaurus no 

 inwardly directed extension of the quadrate has hitherto been 

 observed^, and the discovery of more satisfactory specimens must yet 

 be awaited before it is possible to arrive at any definite conclusion. 



' A. Giinther, " Contribution to the Anatomy of Hatteria {Ehynchocephalus, 

 Owen)," Phil. Trans. 1867, p. 599, pi. xxvi. figs. 3, 4. 



^ H. Gr. Seelej, " Similitudes of the Bones in the EnaUosauria," Jcum. Liun. 

 Soc. (Zoology) vol. xii. (1876), p. 309. 



