646 PEOF. H. G. SEELEY ON THE SKULL OE 



thyosaurus. But if the nasal bones had not been prolonged in front 

 of the nares, and the prem axillary bones had remained merely- 

 bounding their anterior margin, the resemblance of plan between 

 the Crocodile and Ichthyosaur would have been more striking. 

 However, though the long prenarial snout of Ichthyosaurus consti- 

 tutes an excellent ordinal character, because it is constant, it does 

 not appear, from an anatomical point of view, to be of much import- 

 ance a's a measure of an animal's affinities, because it depends on the 

 development of the premaxillary at the expense of the maxillary 

 bone. Important as are the differences of detail, there is a closer 

 family relationship between Ichthyosaurus and living Crocodiles than 

 between Ichthyosaurus and any other existing order of reptiles ; and 

 this resemblance in the skull is also faintly indicated in the ver- 

 tebral column by the double-headed articulation for the ribs, which 

 is not met with among true reptiles, and is more important as a mark 

 of affinity than the development or absence of transverse processes 

 when taken in connexion with skull- characters. 



The only species to which the fossil described makes a near 

 approach in proportionate width of the head is a large form from the 

 Lias of Whitby, which Professor Owen had distinguished under the 

 MS. name Ichthyosaurus crassimamcs, and which the Eev. J. E. Blake, 

 P.G.S., has briefly noticed in the ' Yorkshire Lias,' p. 253. Mr. Blake 

 gives the head as 6 feet 3 inches long, and 3 feet wide behind, but 

 without further description. So far as I remember, the skull in that 

 specimen is greatly crushed, and its characters are obscured, so that a 

 detailed comparison cannot well be made ; but the impression left 

 on my mind by studies made 16 years ago was to the effect that the 

 large animal at York was distinct from the smaller species now de- 

 scribed. I am acquainted with no other Ichthyosaur in which the 

 skull attains this broad, triangular form, with the orbits so far apart 

 from each other and so moderately inclined^ ,and with the nares so 

 far in front of the orbits and relatively so large. The bones pos- 

 terior to the orbit are also more than usually broad. The quadrate 

 bone too descends a good deal below its ordinary position. These 

 characters sufficiently distinguish the species described from all 

 others. 



I would express my grateful thanks to Professor Hughes for his 

 kindness in allowing me to describe and figure this specimen. 



EXPLANATION OF PLATE XXY. 



Ichthyosaurus Zetlandicus. 



Figs. 1-3 are rather less than one fourth natural size. 

 Fig. 4 is reduced one half. 



Fig. 1. The skul] viewed from above. The premaxillary bones are imperfect in 

 front. 



2. The same skull seen from the side, showing the elevated head and large 



orbit. 



3. Palatal aspect of the same skull. 



