50 NATURAL SCIENCE [January 



part of the trunk. No sharp line of demarcation could have been 

 observed externally between the neck and the trunk. 



In the representation of the trunk and the limbs given in the 

 figures already mentioned, there is scarcely anything to alter ; but 

 the Plcsiosaurus guilelmi imperatoris now described, permits an 

 important addition to the end of the body. Sollas has already 

 observed remains of the skin in Plesiosaurus conyhearei. He found 

 it lying upon a part of the dorsal vertebrae and their ribs, and ex- 

 tending as a narrow band over the ends of the neural arches ; but 

 the specimen investigated by him showed no trace of a tail fin. It 

 is to be remarked that R. Owen inferred the original presence of a 

 tail fin from the form of the caudal vertebrae, and this idea is now 

 confirmed. It is determined from the parts of the skin preserved 

 that the Plesiosauria, like the Ichthyosauria, possessed a vertical 

 dermal fin at the end of the body. More nearly complete dis- 

 coveries in the future will perhaps definitely determine its size and 

 shape. The accompanying illustration of the Swabian species (Plate 

 III.) represents it as it seems to me probable from the fragments at 

 my disposal. If the parts go together, as I suppose, its length is 

 approximately given ; but whether the height is correct cannot yet 

 be decided. To justify the rhombic form given to it, it may be 

 pointed out that among all the reptiles and amphibia of the present 

 day which possess a vertical expansion of skin at the end of the 

 body, such as crocodiles and salamanders, it ends in a point, and is 

 never split into two lobes like the ordinary tail fin of a fish. It 

 follows from this that, if the vertebral column remains quite straight, 

 the fin may thus be divided equally between the upper and lower 

 side. It is different among the fishes and Ichthyosauria, in which 

 the vertebral column is bent upwards or downwards at the end ; 

 and among the teleostean fishes it is not made of a single piece, but 

 results from the fusion of the tail fin and the posterior anal fin. The 

 outwardly homocercal caudal fin is also inwardly heterocercal, and 

 its division into two lobes is in direct relation to this. Plesiosaurus, 

 however, with its absolutely straight tail can thus have possessed 

 only a pointed, undivided, dermal fin, and hence the restoration 

 given here. 



This restoration shows yet another difference from those of 

 earlier authors. If the determination of the facts given above is 

 correct we can no longer represent the Plesiosauria as creatures 

 living on the surface of the sea like swimming birds, but they must 

 have lived, like the Ichthyosauria, beneath the surface of the 

 water. For such a mode of life a pointed anterior end of the body 

 is important, and this is here brought about by the elongation of the 

 neck and diminution of the head, while on the other hand the 

 Ichthyosauria have entirely lost the neck, and therefore extended the 



