CAUDAL VERTEBRA. 23 



ihrer Lange iiber den Wirbelkorper hinaus, um mit ihrer fast vierseitigen Endigung dem 

 Knochenwulste des Kreuzbeins so nahe zu treten, dass dadurch eine besonders feste Ver- 

 bindung moglich wurde." 



That vertebra of Pteranodon's synsacrum whose long backwardly directed transverse 

 processes mark the anterior limits of the renal depressions bears the same relation to 

 the three following vertebrae as does the twenty-second vertebra of Quenstedt's specimen 

 to the two vertebrae which follow it. Obviously, the two united vertebrae of Pterodactylus 

 sueoicus supporting the iliac bones are true sacrals, for the following vertebrae have no 

 pelvic connection. Applying the terminology of the simpler case to the more complex, 

 it is evident that the fifth, sixth, and seventh vertebrae of Pteranodon's synsacrum are 

 to be considered true sacrals rather than the two following, whose place in the series 

 so closely resembles that of the two " posterior parapophysial vertebrae " regarded by 

 many recent authors as the true avian sacrals. The definition of the sacral vertebrae 

 used by such authors can not then be applied to the extremely birdlike Pteranodon, 

 and it seems best to retain for this highly specialized pterosaur the simpler definition 

 used by Professor Huxley. 



Four other fragmentary sacra preserved in the Marsh Collection are figured here. 

 Three of these, Nos. 2451, 2570, and 2616 (Plate XII, figures 1, 2, and Plate XIII, figure 2) 

 closely resemble the sacrum of P. ingens. No. 1175, except in their dimensions, which 

 are much smaller. As they are unaccompanied by other skeletal parts of taxonomic 

 value, they may belong to any of the three species of Pteranodon, and no useful ends 

 would be attained by arbitrarily giving them specific names. Two of these specimens 

 have been already alluded to in the discussion of the dorsal vertebras. Plate XIII, 

 figure 3, shows the sacrum of Pteranodon sp., No. 2489, to which reference has also been 

 made in the preceding pages. Great interest attaches to this specimen because of the 

 reduced number of its vertebral parts. The synsacrum is here composed of only nine 

 vertebrae, and as stated above it is not the fourth vertebra, but the third of the series, 

 which sends out the stout transverse processes forming the anterior bounds of the renal 

 depressions. So little is preserved of the rest of the skeleton of No. 2489 that it is 

 quite impossible to determine the taxonomic value of this marked variation. This con- 

 dition is of course more primitive than that seen in the other sacra, and indicates how 

 near the sacrum of Pteranodon may approach that of the allied genus Nyctosaurus, 

 figured by Professor Williston (On the Osteology of Nyctosaurus'), where the second syn- 

 sacral vertebra bears the stout transverse processes. 



It is important to note that the anterior synsacral vertebra is in every case firmly 

 anchylosed to the following member, whether there are ten vertebrae in the series or 

 only nine; the first vertebra included in the synsacra of Nos. 1175, 2451, and 2570 can 

 not then be considered as an insecurely attached unit, which may or may not retain 

 its connection with its fellows according as the local conditions at the time of interment 

 were quiet or disturbed. The coalescence of the synsacrals must have taken place early 

 in the development of the individual. 



CAUDAL VERTEBRAE. 



The number of caudal vertebrae in Pteranodon is not definitely known, but the opinion 

 of Professor Marsh and Professor Williston expressed in various papers, that the tail 

 MEMOIRS CONN. ACAD., Vol. II. 



