1 1 24 



CLASS REPTILIA. 



articulation of the epipodial bones, they present distinct concavities 

 (fig. 1026 bis) for their reception. 



Family Ichthyosaurid^e. — Although Dr Baur makes the genera 

 Mixosaurus, Ichthyosaurus, and Opthalmosaurus the types of as 

 many families, yet it seems on the whole more convenient to 

 include all the known forms in a single family. The most general- 

 ised group with which we are at present acquainted is the genus 

 Mixosaurus, from the Upper Trias of Italy, which is founded upon 

 a small species presenting the following characters. The humerus 

 and femur articulate distally with two bones ; and the radius and 

 ulna are elongated, and separated from one another by an interval 

 throughout their entire length. The teeth are small, and not ex- 

 cessively numerous. It may eventually prove that all the Triassic 

 forms are referable to this genus. The type 

 genus Ichthyosaurus is characterised by the usual 

 presence of teeth throughout the jaws ; and by 

 the normal articulation of the humerus with only 

 the short radius and ulna, which are in close ap- 

 position (fig. 1027). In the pelvic limb the 

 femur similarly articulates only with the tibia 

 and fibula at its distal extremity ; and thus this 

 extremity in both the humerus and femur has only 

 two articular facets (fig. 1026 bis). The bone 

 which articulates inferiorly with the radius and 

 ulna at their junction is the intermedium (fig. 

 1027, i) j while the one filling up the angle be- 

 tween the latter and the radius is the radiale ; 

 the opposite angle being occupied by the ulnare. 

 Below the intermedium we have the centrale, 

 which in the more specialised forms may be 

 double (fig. 103 1), and below this the remain- 

 ing carpals, metacarpals, and phalangeals, which 

 are combined to form a pavement-like structure. 

 The bones forming the continuation of the line 

 of the intermedium in fig. 1027 correspond to 

 the middle or third digit of the typical manus ; 

 and the structure of this limb indicates that it 

 was derived from a four-fingered ancestor, since the pollex, or first 

 digit, is not represented. 



&8* 



Fig. 1027. — Dorsal 

 aspect of the left pec- 

 toral limb of Ichtliyo- 

 saurus latifrons ; from 

 the Lower Lias. Re- 

 duced, h, Humerus ; 

 r, Radius ; «, Ulna ; i, 

 Intermedium. (After 

 Owen.) 



Specimens have been obtained from the Lias of Wiirtemberg and 

 Barrow-on-Soar, in Leicestershire, which show the contour of the in- 

 teguments of the paddles. It appears from these that on the anterior 

 border of the paddle (fig. 1027 bis) there was a comparatively narrow 

 band of integument, which was covered by minute horny scales ; while 

 on the posterior border there was a much broader flap, which appears 



