MKKKIA.M: TRIASSIC ICHTIIYOSAURIA. 67 



posteriorly, having expanded somewhat on this side, as in the limbs of most 

 aquatic forms. In Cymbosponfyliis the posterior border is expanded, but 

 still shows a portion of the median or shaft region that has not been filled out 

 by lateral expansion. 



In the posterior limb of Mi.n>*<ii<n<x the propodial and epipodial bones are 

 more slender, and more strongly constricted in the median region than are the 

 corresponding elements of the anterior limb. In Cymbospondylus the femur 

 is a little more slender than the humerus; the tibia is narrower distally than 

 the radius, and the fibula has much the same form as the ulna. 



In Mi.r<iMiun{fs the proximal row of the mesopodial region in the anterior 

 limb consists of four elements of which the anterior three are the radiale, 

 intermedium, and ulnare, in their normal positions. The fourth is a large 

 pisiform set far up against the posterior margin of the ulna. In the typical Cym- 

 l>oNi>(i(li/lnn three proximal elements are known but the form of the posterior 

 side of the ulna suggests the presence of a pisiform as in Mixosaurus. 



In Mi.roNmirus the podial region consists of five fingers, the number of 

 phalanges in each of which is relatively small for an ichthyosaurian, not ex- 

 ceeding ten to eleven in the median digits. Many of the individual phalanges, 

 particularly those in the proximal half of the phalangeal region and corre- 

 sponding more nearly to the primitive phalangeal elements, are slightly elon- 

 gated and show notches on one or both lateral margins, of such a form as to 

 suggest an originally constricted shaft region. 



In Cymbospondylus the podial elements have never been seen in complete 

 series, but in several instances specimens representing parts of limbs show 

 groups of three ossicles in rows, suggesting that possibly we are dealing here 

 with transverse sections through mesopodial or podial regions with three digits. 

 The borders of the rounded elements are not excavated or pitted as in Shastn- 

 w(i(ru>i. (See figs. 112 and 113.) In one specimen an element similar to the 

 doubly notched phalanges of Mixosaurus is associated with limb bones. 



Considering the common characters of the limbs of these two genera, the 

 oldest of the ichthyosaurian forms, both are seen to have more primitive epi- 

 podials than appear in any of the later genera. There is also some suggestion 

 that in Cymboapondylus, as in Mijcosaurus, a largely developed pisiform ele- 

 ment was present and in articulation with the posterior margin of the ulna. 

 In so far as they are known to differ, Cymbospondylus, apparently the older 

 genus, is more primitive, the humerus being more slender and longer in com- 

 parison with the longitudinal diameter of the mesopodials; while the ulna 

 remains narrower through failure to close the posterior notch by increase of 

 the transverse diameter. Excepting those more primitive features of ('i/nibo- 

 nf)(i<l/j/itN mentioned above, the limbs of Mixosatirus appear to be the most 

 primitive ichthyosaurian extremities known, and may serve as representatives 



