The Reptiles and Birds at first sight seem to be widely different. The latter are the warmest blooded of all vertebrates, the former are coldblooded. The one wear feathers, the other scales. Nevertheless, there is an intimate connection between them; the reader has doubtless already learned from other sources the facts about their relationship, so we will not here do more than recall a few of these facts. One is, that the birds of earlier times had teeth in their beaks, and possessed jointed tails. Another, that the Reptiles of earlier times included forms that were able to fly. A third notable fact is the presence of claws on the wings of some birds, showing that the wing of the bird was not always wholly specialised for use in flight. We owe to Professor Huxley, the recognition of the close relationship of Birds and Reptiles, and the name Sauropsida (Reptile-like animals), under which both are included. They agree in being air-breathers and never having gills, except the rudiments present in the early embryo: this distinguishes them from Amphibia. They agree in having the skull set on to the back-bone by a single articulating surface or condyle; and thus differ alike from Amphibia and from Vertebrata. They agree in having the red corpuscles of the blood nucleated; and in this differ from the Mammalia, in which the red corpuscles are non-nucleated discs. From a popular point of view, we may say that the striking distinction between birds and reptiles lies in beauty and ugliness. Even in their eggs, the reptiles display no love for adornment, no colouring or pattern. Fig. 44 shows the eggs of some reptiles. [Illustration: FIG. 44.--Eggs of Reptiles, half the natural size. _A_, of African Cobra. _B_, of Common English Snake. _C_, of Common English Lizard, _Lacerta agilis_. _D_, of Elephantine Tortoise. _E_, of Crocodile.] The five chief groups of existing reptiles are the Chelonia (Tortoises and Turtles); the Rhyncocephala, represented only by _Hatteria_, a lizard found in New Zealand; the Lacertilia or Lizards; the Ophidia, or Snakes and Serpents; and the Crocodilia. Perhaps the most interesting point regarding the reptiles that can be mentioned in brief space, is the fact that they present traces of a median third eye, which have been described by Baldwin Spencer, in the New Zealand Hatteria, and in other reptiles. It is situated on the roof of the brain. While the structure in Hatteria shows it to be an eye, its position corresponds with that of the pineal gland of vertebrates generally; so that we find, in fact, the trace of a third eye in all vertebrates, including ourselves. It is, however, a trace only. In the Lamprey fishes as well as in _Hatteria_, it reaches a further degree of development. This pineal eye has been compared in structure to the eye of Ascidians.