50 
POPULAR SCIENCE REVIEW. 
losaurus, for certain living Crocodiles have the lateral ridges 
of the teeth marked with minute serrations. Professor Huxley 
lias shown that all Dinosaurian teeth, no matter how worn 
ultimately, are characterized by more or less marked serrations 
of the compressed margin. The teeth in front are frequently 
simple and conical incisors, contained in the pre-m axillary 
hone, which do not become worn down, because they appear to 
have had adapted to them a toothless pad in the lower jaw, 
which was hollowed out something like the beak of a parrot. 
Behind these, without any intervening canine tooth, so far as 
known forms go, succeed the molars, which in the herbivorous 
genera usually have their surfaces worn more or less flat. 
The distinction into pre-molars and molars is a character fairly 
marked in the Crocodilian jaw, where the canine teeth also 
may be detected ; though, owing to the molars not usually 
being worn down, they do not present the modified appearance 
seen in Dinosaurs. The resemblances perhaps are equally close 
in the teeth of Lizards, where pre-molars and molars are often 
easily recognized, though some Lizards, like the Cnemidophorus 
lacertinoidcs from Mexico, show distinct incisors, pre-molars, 
and molar teeth with cusps, like those of mammals. Among 
the Reptilia, probably no part of the skeleton is of less 
value than the teeth for purposes of classification ; and the 
same type, if not the exact form, is maintained in nearly 
allied genera. Taking the skull as a whole, and contrasting 
its Triassic forms with those of the Wealden beds, the dif- 
ferences which have developed themselves are in all cases such 
as would be held, in the vertebrate province, to pertain to a 
higher group of animals ; but though in Hypsilophodon the 
eyes rest against hollows in the frontal bones (Fig. 1), as 
among some birds and many mammals, there is nothing in the 
skull which indicates that the Dinosauria are a transitional 
group, passing into either the Mammalian or Avian Class. 
The vertebral column shows a good deal of variety. If 
Professor Owen is correct in identifying as Dinosaurs certain 
reptiles from the Trias of South Africa, the vertebrae may 
have the articular faces of the centrums as deeply biconcave 
as in the living Ilatteria or in Ichthyosaurus. In Zanclodon the 
cervical vertebrae are greatly elongated, like those of a long- 
necked ruminant, and concave at both ends (Fig. 4). In this 
genus, as in Morosaurus, figured by Professor Marsh, the atlas 
and axis are separate bones, as in the existing crocodile, the 
odontoid mass of the atlas fitting firmly, however, against the 
front of the second vertebra. This is the more interesting 
because in some Wealden types, like Iguanodon, the odontoid 
process is anchylosed with the axis, so as to closely approximate 
in appearance to the condition of the axis in a bird (see Fig. 3). 
