ICHTHYOSA URIA . 515 



than in Plesiosaurus. They are divided into three sub-orders, viz., 

 the Dicynodontia, distinguished by two tusks in the position of 

 canine teeth ; the Cynodontia, which have teeth anterior and pos- 

 terior to the canines on the mammalian plan, so that Sir Richard 

 Owen has termed them " Theriodonts ; " and, thirdly, the Crypto- 

 dontia, in which the jaws are edentulous, and teeth, when present, 

 are palatal. This group is chiefly known from South African fossils. 

 Rkynchosaurus and H ~ yperodapedon from the British Trias have been 

 referred by Sir R. Owen to the Cryptodontia. Placodus may be placed 

 here. Many of these Anomodont fossils present so remarkable a re- 

 semblance to the lowest types of mammalia in various parts of the 

 skeleton, that the reptiles seem to foreshadow the higher group. 1 



IchtJiyosauria. The Ichthyosauria have a type of skull which 

 differs from that of Dinosaurs chiefly in the enormous prenarial 

 elongation of the snout, and the less perfect ossification of the skull. 

 The squamose overlapping of the bones is also seen in fishes and 

 Cetaceans, and is connected with aquatic habit. The biconcave con- 

 dition of the vertebral column is closely paralleled in some Dinosaurs 

 from South Africa. The double-headed articulation for the rib is a 

 character only found in association with well-developed ribs in croco- 

 diles and higher vertebrata. The circumstance that the articulation 

 for the ribs descends in position from the neck to the pelvis is asso- 

 ciated with marine habit, and the absence of organs ministering to 

 locomotion on land. The scapular arch is in many respects Dino- 

 saurian ; the pelvic arch is so feebly developed as only to suggest a 

 generalised reptilian type. Many species of Ichthyosaurus were vivi- 

 parous. 2 Few genera have at present been defined. A genus of the 

 Oxford clay has been termed Ophthalmosaurus, in which there are 

 three bones in the forearm, and the clavicle and interclavicle arc 

 united by suture into one mass. A femur from the Cambridge Green- 

 sand has been referred to a genus Cetarthrosaurus, and is regarded as 

 Ichthyosaurian. Professor Marsh has described an American toothless 

 ichthyosaurus named Sauranodon, which, in other respects, closely re- 

 sembles the European type. There is no evolution of the Ichthyo- 

 saurs at present known comparable to that of Dinosaurs. The oldest 

 species are found in the Rhsetic beds, the newest in the Chalk. The 

 remains found in the London Clay are probably derived fossils. 



Dinosauria. The whole of the Dinosauria, so far as is at present 

 known, were land animals, and their remains are most abundant in 

 those formations which give evidence of near proximity to land, such 

 as Trias, Wealden, and Greensand, though they are represented in 

 almost all Secondary deposits. It is probable that Protorosaurus, 

 found in the Permian, and best known as the fossil monitor of Thur- 

 ingia, must be included in the Dinosaurian order. The Trias has 

 yielded (especially from the Bristol conglomerate) remains of Theco- 

 dontosaurus, Palceosaurus, and Teratosaurus ; but the finest mate- 

 rials of this age are found in Wiirtemberg, and belong to the genus 



1 See Catalogue of the South African Reptilia in the British Museum. By 

 R. Owen, F.R.S. 2 Kept. Brit. Assoc., Swansea, 1880. 



