BRITISH FOSSIL REPTILES. 10/ 



into each other at the sides, so as to prevent any independent 

 movement, but to constitute one uniformly resisting framework 

 of a povrerful fin. The integument extended beyond the bones 

 further than might have been anticipated, and its posterior 

 margin appears to have been supported by a series of slender 

 bifurcate cartilaginous rays. A disparity in the size of the 

 anterior and posterior extremities has usually been assigned as 

 a generic structure or character of the Ichthyosaurus ; but in 

 the species in which the head is unusually large, as the Ich. pla^ 

 ti/odon, the pelvic extremities are as large as the pectoral ones. 

 In most of the known species, however, the hinder paddles are 

 much smaller than the fore ones. In all they present a structure 

 which is closely analogous to that above described. 



The Pelvic Extremity . — The pelvic arch consists, as in the 

 Plesiosaurus, of an ilium, ischium and pubis on each side. The 

 ilium is a short, simple, strong, compressed bone, slightly ex- 

 panding as it descends to combine with the ischium and pubis 

 in forming the acetabulum. Its upper and proximal extremity is 

 not connected by synchondrosis to the extremities of the sacral 

 ribs, but lies simply upon them, just as the scapula rests upon the 

 ribs at the anterior part of the thorax. This is a condition of 

 the ilium which is of great interest, and is peculiarly character- 

 istic of the JEnaliosauria among Reptiles*. It renders their 

 pelvic extremities remarkably analogous to the ventral fins of 

 fishes, which are in like manner simply suspended in the mus- 

 cular mass and not fixed to a sacrum. The ischia and pubes 

 are both relatively much smaller than in the Plesiosauriis ; the 

 pubis is slightly expanded at its mesial or lower end, but the 

 ischium is a simple, elongated, slightly compressed bone. 



The femur is usually longer in proportion to its breadth than 

 the humerus, and its posterior margin is more concave. It is 

 succeeded by two bones, which represent the tibia and fibula, 

 and resemble in their forms and proportion the radius and ulna 

 of the fore fin. Three irregular polygonal flattened bones suc- 

 ceed the tibia and fibula ; and then three or four longitudinal 

 series of similar bones follow, which gradually diminish in size 

 as they approach the extremity. 



Such appears to be the structure of the skeleton which is 

 common to all the species of Ichthyosauri, so far as I have been 

 able to study their fossil remains, and which may, therefore, be 

 considered to characterize them generically. 



The modifications of this structure which distinguish the 

 particular species next claim attention. 



* Tho rudimental pelvic exU'emities of Serpents are simply suspended in the 

 muscles external to the adjoining ribs, as were those of the IdUhyosuunis. 



