200 Mr. Owen on the soft parts of the Hind Fin 



of the soft parts of the fin in the slab of lias containing the mutilated paddle of the 

 Ichthyosaurus here described. 



It appears to be the posterior fin of the Ich. communis ; the impressions and frac- 

 tured portions of the two middle series of phalangeal ossicles are nearly complete, 

 and there are the terminal portions of the two outer rows, making six digits in all. 

 The impression, and a thin layer of the dark, carbonized integument of the terminal 

 half of the fin, are most distinctly preserved and exposed on the fractured surface 

 of the lias matrix. The contour of the extremity of the fin is thus clearly defined ; 

 and is accurately represented in the subjoined figure, PL XX. The anterior 

 margin is formed by a smooth, unbroken, well-defined line, and appears to have 

 been merely a duplicature of the integument ; but the whole of the posterior mar- 

 gin exhibits the remains and impressions of a series of rays by which the fold of 

 integument has been supported. Immediately posterior to the digital ossicles 

 there is a band of carbonaceous matter varying from two to four lines in breadth, 

 and extending in an obtuse, pointed form for an inch and a half beyond the di- 

 gital ossicles ; this I take to be the remains of the dense ligamentous matter which 

 immediately invested the bones of the paddle ; its fibrous structure is distinctly 

 displayed. The rays, above mentioned, are continued from the posterior edge 

 of this carbonized ligamentous matter, in which their bases appear to have been 

 implanted : they extend, with a slight curve, obliquely downwards to the edge of 

 the tegumentary impression ; the upper rays are directed more transversely, 

 but they gradually lie more in the direction of the axis of the fin as they approach 

 its termination ; the most interesting feature in these rays is, that they bifurcate 

 as they approach the edge of the fin, the two branches diverging in a very 

 slight degree, and thus repeating, as it were, the character of the digits themselves. 

 From the rarity of their preservation, and their appearance and co-existence in 

 the present instance with remains of the integument, it is evident that they were 

 not osseous, but probably either cartilaginous or of that albuminous horn-like 

 tissue of which the marginal rays consist in the fins of the Sharks and other Pla- 

 giostomous Fishes. Besides the impressions of the posterior marginal rays, there 

 are others of not less interest in the present specimen : there is a series of raised 

 transverse lines, crossing the whole of the fin at intervals of about one-eighth of 

 an inch, having a very slight curve, with the concavity towards the end of the fin. 

 As it is the inner surface of the integument which is exposed to view, — the opposite 

 side of the paddle to that which is imbedded in the lias having, with portions of 

 the digital ossicles, been broken away, — these fine transverse, raised lines probably 

 indicate corresponding impressions on the exposed surface of the integument, and 

 from their regularity I conclude that there was a division of the rigid integument 

 into scutiform compartments analogous to those on the paddle of the Turtle and 



