210 



PKOF. H. G. SEELEY ON A SACKUM OE A 



large A. Forsteri, which is not dissimilar in size, there is enough in 

 common with the fossil to make some comparisons instructive. 



In the skeleton of Ajptenodytes Forsteri, in the Osteological Col- 

 lection of the British Museum, numbered 1197 6, the presacral ver- 

 tebra is imperfectly anchylosed to the sacrum, and comes away 

 leaving a rugose surface, which is slightly convex from the ventral 

 margin to the neural canal. This is the converse condition to that 

 seen in the fossil. But just as birds have the vertebrae biconcave, 

 flat, and opisthoccelous, as well as saddle-shaped in the intercentral 

 articulation, there is no antecedent improbability in an extinct bird 

 having the articulation procoelous. 



This Penguin's sacrum contains ten vertebrae, and differs from 

 the fossil in many details. It has not the median ventral canal, 

 nor the pneumatic pits ; but the Penguin is exceptional among water- 

 birds in that respect. The transverse processes of the early sacral 

 vertebrae of the Penguin are inclined forward as they ascend from 

 the centrum to the neural platform, so that the process rises from one 

 centrum to the neural arch in front of it ; whereas in the fossil the 

 processes are vertical, and given off at the junction of two centrums. 

 The middle three sacral vertebrae of the Penguin, have the character 

 already discussed as typical of existing birds, the attachments for 

 the ilium being given off from the neural arch, and this is an 

 important difference from the fossil. But the posterior widening of 

 the transverse processes seen in the Penguin is well marked in the 

 fossil. And the last vertebra has the vertically ovate tubercle for 

 the ilium upon its own centrum, and is conditioned in every respect 

 in the same way in the Penguin (fig. 6) and the fossil (fig. 5). The 

 correspondence is almost as close in the character of the transverse 

 processes and zygapophyses of the first sacral vertebra. The resem- 

 blances further include the flattened neural platform, which is better 

 developed in the fossil than in the Penguin. The neural spine in the 

 fossil is thinner, more as in ordinary birds, and apparently lower than 

 in the Penguin. Another resemblance is in the descent of the trans- 

 verse processes from the neural platform in the first vertebra to the 

 base of the centrum in the fourth vertebra, in consequence of which 

 the first two vertebrae have their bodies compressed from side to side 

 (fig. 3). In both types the bodies of these vertebrae are of similar 

 form, the first rather flattened on the underside and concave in 

 length, the second widening as it extends posteriorly. Though the 

 centrums in the fossil are approximately of equal length, their length 

 diminishes in the middle of the sacrum in a way which is Avian, 

 though the shortening is less than in existing birds. Prom the Avian 

 form of the last sacral vertebra it may perhaps be legitimately 

 inferred that the tail was not unlike that of a Penguin. 



Prom this comparison it is manifest that the differences which the 

 fossil shows from existing birds are three : first, the small number 

 of vertebrae in the sacrum ; secondly, the absence of the sacral 

 recesses for the middle lobes of the kidneys ; and, thirdly, the form 

 of the articular face of the first sacral vertebra. Possil birds lessen 

 the importance of these differences : first the Archeeopteryx has as 



