BIRD FROM THE WEALDEN OE BROOK. 



209 



(fig. 2, vc) seen in the median line of this fossil, which is better com- 

 pared with the similar groove seen on the sacral centra in Vultures 

 and Ichthyornis victor. If we were to say that the general analogy 

 with Dinosaurs is such as to give a reasonable probability that some 

 member of the group will be found to resemble it, the prediction 

 would not seem inadmissible ; but it would only be another mode of 

 recognizing the approximation of the sacrum in Dinosaurs towards 

 that structure in birds. Unfortunately nothing is known of the 

 sacrum in British Dinosaurs with pneumatic vertebrae ; but if the 

 American types which are thus characterized may be taken in 

 evidence, the sacrum has nothing in common with the sacrum 

 described which would tend to affiliate it to the Dinosauria. 



The sacral vertebrae in Ornithosaurs are also united into a sacrum. 

 Von Meyer has stated the number of vertebrae in species from the 

 Solenhofen Slate at from 5 to 6. They are anchylosed at a suffi- 

 ciently late period for the vertebrae not to show the diminished 

 length seen in the sacrum of a bird. There is a resemblance to 

 the fossil in the transverse form of the articular face of the centrum, 

 a less resemblance to the concave articular surface of the first sacral 

 vertebra, and no resemblance to the flat or concave articulation of 

 its last vertebra, which in Cretaceous Ornithosaurs is convex. 

 There is a general resemblance in the pneumatic character of the 

 bones ; but I am not aware that any Ornithosaur has the foramina 

 similarly situate, or that the structure of the sacrum is such as this 

 fossil shows. 



Turning to birds, the form of the articular face of the first sacral 

 vertebra is variable, so that the Gannet, which has the lower dorsal 

 vertebrae almost biconcave, has the articular face flat, with a central 

 pit in the first vertebra of the sacral mass, which cannot be termed 

 sacral, since it carries ribs. There is therefore no insuperable 

 difficulty in the absence of the typical Avian articulation of the 

 first vertebra in interpreting the sacrum as that of a bird, since the 

 Avian articulation has already been found to be absent in the sacrum 

 of Enaliornis from the Cambridge Greensand, and in that of Ichthy- 

 ornis, figured by Professor Marsh. 



In Ichthyornis dispar * there are ten vertebrae in the sacrum ; but 

 the specimen forms an interesting link with our fossil, in differing 

 from existing birds by not having the transverse processes of the 

 middle sacral vertebrae elevated from the centrum on to the neural 

 arch. It is true that they rise a little on the last three sacral vertebrae 

 of Ichthyornis dispar, but then there are more vertebrae in the sacrum 

 than in this fossil. Ichthyornis, however, is no near ally. 



Among existing birds, I was led to seek an elucidation of the 

 fossil among the Penguins rather than any other type, because their 

 dorsal vertebrae present the opisthocoelous deviation from the Avian 

 articulation, which has already offered a suggestive explanation of 

 the opisthocoelous articulation in certain vertebrae of some Dinosaurs, 

 towards which the sacrum may approximate in a general way. The 

 sacrum of Aptenodytes, however, is typically Avian. Yet in the 



* Ichthyornis victor may be generically distinct. 



