1873.] EGERTON — TWO FOSSIL PISHES. 419 



joints, that pressure had also assisted in rendering this structure 

 more visible, and that the structure itself might he accounted for 

 by a combination of the forces of crystallization and mechanical 

 pressure. 



3. On Peatysiagum scleeocephalum, Egerton, and Paljeospinax 

 peisctts, Egerton. By Sir Philip Geey Egerton, Bart., M.P., 

 E.R.S., F.G.S. 



In the 13th Decade of the Memoirs of the Geological Survey of the 

 United Kingdom, published in 1872, I described the above-named 

 fossil fishes from the Lias formation of Lyme Regis. Since the issue 

 of that publication I have obtained specimens elucidating parts which 

 were defective in the former examples, and which I consider of 

 sufficient importance to warrant description. On referring to the 

 figure of the former (Platysiagwri), given on plate 6 of the Decade, 

 it will be seen that the dorsal and anal fins are wanting. This also 

 happens with the several specimens described in the text but not 

 figured. These desiderata are furnished by a fine specimen recently 

 obtained. The two extremities are wanting ; the remainder of the 

 fish measures 15-i inches in length. It corresponds very closely in 

 size with the larger of the two specimens described in the Decade. 

 The total length of the fish, if restored, would probably be about 

 22 inches. The vertical diameter of the trunk is very uniform 

 between the occiput and the dorsal fin, and measures 4 inches. This 

 is a singular character in a Sauroid fish, in which family the body is 

 generally fusiform, tapering gradually from the thoracic arch to tho 

 commencement of the caudal fin. The dorsal fin which is here pre- 

 served is very singular both in position and structure. It is situated 

 9i inches behind the occiput, and occupies a position vertically opposed 

 to the interval between the ventral and anal fins, and extends thence 

 nearly to the origin of the tail. This remote position of the dorsal 

 fin is very unusual in the Sauroid fishes of this age. The organ is 

 composed of 17 or 18 fin-rays occupying a space on the back of one 

 inch and a half. Three or four of the first rays are dermal, in- 

 creasing gradually in length, and acting as fulcra to the true 

 anterior rays of the fin. The first of the latter carries a fringe of 

 ossicles on its margin ; and this feature is continued on the second ray 

 from the point at which it exceeds tho length of the first ray. The 

 following rays composing tho fin have expanded bases for attachment 

 to the interspinous ossicles. The shaftsforsome little distance are single 

 and angular; they then take on a flattened character and are traversed 

 by numerous cross joints, and as they recede from the base have succes- 

 sive bifurcations. The most remarkable feature in the structure of this 

 organ is this : at each scission of the fin-ray the posterior limb breaks 

 up into a tuft of ray lets, while the anterior portion lengthens singly 

 until the next bifurcation, where a similar process is repeated. This 

 is a structure I am not acquainted with in any other genus of fossil 

 fishes, although an approach to it occurs in the fins of the genus 



