The great German delineator of these animals, Von Meyer, admitted six different species. Mr. Newton and Mr. Lydekker diminish the number to four. It is not easy to determine these differences, or to say how far the differences observed in the bones characterise species or genera. It is certain that there is one remarkable difference from other and older Pterodactyles, in that the last or fourth bone in the wing finger is usually slightly longer than the third bone, which precedes it. There is a certain variability in the specimens which makes discussion of their characters difficult, and has led to some forms being regarded as varieties, while others, of which less material is available, are classed as species. I am disposed to say that some of the confusion may have resulted from specimens being wrongly named. Thus, there is a Rhamphorhynchus called curtimanus, or the form with the short hand. It is represented by two types. One of these appears to have the humerus short, the ulna and radius long, and the finger bones long; the other has the humerus longer, the ulna much shorter, and the finger bones shorter. They are clearly different species, but the second variety agrees in almost every detail with a species named hirundinaceus, the swallow-like Rhamphorhynchus. This identification shows, not that the latter is a bad species, but that curtimanus is a distinct species which had sometimes been confounded with the other. While most of these specimens show a small but steady decrease in the length of the several wing finger bones, the species called Gemmingi has the first three bones absolutely equal and shorter than in the species curtimanus, longimanus, or hirundinaceus. In the same way, on the evidence of facts, I find myself unable to join in discarding Professor Marsh's species phyllurus, on account of the different proportions of its limb bones. The humerus, metacarpus, and third phalange of the wing finger in _Rhamphorhynchus phyllurus_ are exceptionally short as compared with other species. Everyone agrees that the species called longicaudus is a distinct one, so that it is chiefly in slight differences in the proportions of constituent parts of the skeleton that the types of the Rhamphorhynchus are distinguished from each other. I cannot quite concur with either Professor Zittel (Fig. 58, 3) or Professor Marsh (Fig. 58, 2) in the expansion which they give to the wing membrane in their restorations; for although Professor Zittel represents the tail as free from the hind legs, while Professor Marsh connects them together, they both concur in carrying the wing membrane from the tip of the wing finger down to the extremity of the ankle joint. I should have preferred to carry it no further down the body than the lower part of the back, there being no fossil evidence in favour of this extension so far as specimens have been described. Neither the membranous wings figured by Zittel nor by