373 THE ANIMAL KINGDOM. 



SAUROPSIDA. 



AIR-BREATHING Vertebrata, with a skin remarkably deficient in glands 

 and an epidermic skeleton in the shape of feathers, scales, or scutes. The 

 skull articulates with the vertebral column by means of a single occipital 

 condyle into which the basi- and ex-occipital bones enter in varying pro- 

 portions. The pro-, epi-, and opisth-otic bones either remain separate inter 

 se, and fuse with adjoining bones, or else fuse with adjoining bones and 

 then with each other about the same time (Birds). There is an interorbital 

 septum and the carotids pierce the basisphenoid, entering the cranial 

 cavity at the pituitary fossa. The mandible is always complex : each 

 ramus consisting of one cartilage bone, the articular, and five membrane 

 bones, the dentary, splenial, coronoid, angular and surangular. It articu- 

 lates with the skull by the intermediation of the upper part of Meckel's 

 arch, the quadrate bone, which is free or fixed. There are well-developed 

 cervical ribs, and the neck passes insensibly into the thorax. The sternal 

 elements of the ribs unite on each side in the embryo to form a broad plate 

 on the right and left side of the body, which are always in contact and with 

 rare exceptions fuse in the middle line : and either remain as cartilage more 

 or less ossified, or are eventually replaced by membrane bone. When the 

 pelvis is present, the true axis of the ilium trends forwards and downwards. 

 There are as in all lower Vertebrata no epiphyses to the bones. There is 

 no corpus callosum. The olfactory nerve forms a single strand invariably. 

 The cervical sympathetic cord is generally double. The ciliary muscle of 

 the eye is transversely striated and the cones of the retina contain clear and 

 variously coloured oil globules except in Opkidia, the Crocodile and 

 Geckoes. There is a cloaca (= proctodaeum) common to the rectum and 

 urino-genital ducts. The heart has two auricles and either one or two ven- 

 tricles, but the single ventricle is physiologically divisible into two. The 

 haematids are oval and nucleated and smaller than in Ichthyopsida. The 

 urine is semisolid and contains urates and not urea. The ova have a single 

 layer of cells in the tunica granulosa of the Graafian follicles : they are 

 large and projecting from the ovary when ripe. The oviduct has entire 

 edges as in all lower Vertebrata to its abdominal aperture, and is divisible 

 into a narrower conducting portion, a glandular portion in which albumen 

 is secreted, and a lower muscular or uterine portion in which the ovum stays 

 while the shell is formed. Accessory glands to the reproductive organs are 

 nearly invariably absent. The egg-shell is more or less calcareous. All 

 are oviparous with rare exceptions among the Reptilia. The ovum is 

 telolecithal and segmentation consequently partial. 



There are two classes, Aves and Reptilia. 



