The great difference between the sacrum of a Pterodactyle and that of a bird has been rendered intelligible by the excellent discussion of the sacral region in birds made by Professor Huxley. He showed that it is only the middle part of the sacrum of a chicken which corresponds to the true sacrum of a reptile, and comprises the five shortest of the vertebræ; while the four in front correspond to those of the lower part of the back, which either bear no ribs or very short ribs, and are known as the lumbar region in mammals, so that the lower part of the back becomes blended with the sacrum, and thus reduces the number of dorsal vertebræ. Similarly the five vertebræ which follow the true sacral vertebræ are originally part of the tail, and have been blended with the other vertebræ in front, in consequence of the extension along them of the bird's hip bones. This interpretation helps to account for the great length of the sacrum in many birds, and also explains in part the singular shortness of the tail in existing birds. The Ornithosaur sacrum has neither the lumbar nor the caudal portions of the sacrum of a bird. THE TAIL The tail is perhaps the least important part of the skeleton, since it varies in character and length in different genera. The short tails seen in typical pterodactyles include as few as ten vertebræ in _Pterodactylus grandipelvis_ and _P. Kochi_, and as many as fifteen vertebræ in _Pterodactylus longirostris_. The tails are more like those of mammals than existing birds, in which there are usually from six to ten vertebræ terminating in the ploughshare bone. But just as some fossil birds, like the Archæopteryx, have about twenty long and slender vertebræ in the tail, so in the pterodactyle Rhamphorhynchus this region becomes greatly extended, and includes from thirty-eight to forty vertebræ. In Dimorphodon the tail vertebræ are slightly fewer. The earliest are very short, and then they become elongated to two or three times the length of the early tail vertebræ, and finally shorten again towards the extremity of the tail, where the bones are very slender. In all long-tailed Ornithosaurians the vertebræ are supported and bordered by slender ossified ligaments, which extend like threads down the tail, just as they do in Rats and many other mammals and in some lizards.