2620 



THE REMAINING GROUPS OF REPTILES 



show the nearest relationship to the Kgg-laying Mammals, and until we know the 

 true affinity of the latter to the other members of the same class, it is of course 

 impossible to attempt to define the genealogy more exactly. The Anomodonts 

 are the only reptiles which agree with the Egg-laying Mammals in having 

 three distinct bones on each side of the true shoulder-girdle; that is to say, 

 a blade bone, or scapula, above, and a coracoid and metacoracoid below. Then 

 the pelvis is very mammal-like, not only in that its three elements are united, but 

 likewise in the small size of the vacuity, or foramen (of) between the pubis and 

 ischium. It will also be seen from the two figures here given how close is the re- 

 semblance between the pelvis and shoulder girdle of these reptiles, each having one 

 bone above and two below. Even still more marked is the similarity between the 

 upper arm bone or humerus of the Anomodonts and that of the Egg-laying Mam- 

 mals; each having a perforation on the inner border of the lower end, whereas in 

 those existing reptiles which possess such a perforation (with the exception of the 

 tuatera, where there is one on each side), it is situated on the outer border. As a 

 rule, the Anomodonts further resemble Mammals in the absence of abdominal ribs; 

 and there are important similarities in the structure of the skull. 



Anomodonts are met with in the Triassic rocks, and are represented by at 

 least four well-marked subordinal types. In the first group, known as Mammal- 



UPPER SURFACE OF THE SKUU, OF A 



PAVEMENT-TOOTHED ANOMODONT. 



(One-third natural size.) 



SKULI. AND TEETH OF GALESAUR. 



toothed (Theriodont) Reptiles, the teeth, 

 as exemplified in the figure of the skull 

 of the African galesaur, are differentiated 

 into incisors, tusks, and cheek teeth; the 

 latter frequently having three cusps ranged 

 in a longitudinal series. Whether, how- 

 ever, this marked mammalian type of 

 dentition is indicative of genetic affinity 



with Mammals, may be opened to doubt, as it is quite as likely to be due to paral- 

 lelism in development. Another modification is presented by the Dicynodonts of 

 England, Africa, and India, in which the jaws formed a horny beak, either destitute 

 of teeth, as in the tortoises, or provided with a huge pair of tusks in the upper jaw; 

 some of these reptiles being of gigantic size. A third group, known as Pavement- 

 toothed, or Placodont Reptiles, which should probably be included in the order, are 

 characterized by the presence of broad, flattened teeth on the palate and jaws, as 

 shown in the figure on p. 2361; the skull being very short and more or less 



