MKRRIAM: TRTASSIO TriTTTIYOSAFRIA 71 



almost to come in contact with the tibia. In the specimen figured by Repossi, 

 this element of the second row seems to have reached the tibia. This expla- 

 nation not only accounts for the fourth element of the proximal row, but may 

 also explain the presence next the posterior border of tarsale one, and appar- 

 ently functioning as tarsale two, of an element showing a strong median 

 constriction. This bone has the appearance of a metapodial or phalange, and 

 if the explanation given above is correct, is really a metapodial moved upward 

 into the position of tarsale two. 



Judging from the other relationships indicated in the characters of Mi.ro- 

 xanntx there seems to be reason for considering that the genus is possibly not 

 ancestral to any form of Ichthyosaurus, but is associated more closely with 

 those forms in which the inferior rib head has been almost entirely lost. If 

 such be the case, the broad type of paddle in Miiro.su uni.s is evidently an inde- 

 pendent expression of a tendency similar to that shown in the broad-paddled 

 forms of Ichthyosaurus. 



Men-lamia. The only Upper Triassic form in which the structure of the en- 

 tire limb is known is the genus Men-lamia, from the upper portion of the Upper 

 Trias of California. In this type the maims (fig. 92) is relatively slender and 

 includes only three functional digits with the faint rudiment of a fourth be- 

 hind these. The humerus is relatively slender and shows a median constriction. 

 The radius and iilna are one-half as long as the humerus and are separated 

 by a large opening. Beyond the first row of the carpus, the limb elements are 

 arranged in three well defined linear series, between which there is no inter- 

 locking. The metapodial and phalangeal elements are all more slender than 

 in Iclitliyosaia-u*. The elements of the two outer rows are all notched on the 

 outer border, and in many instances a median constriction is developed. In 

 the middle digit lateral notches are present only on the posterior border of 

 the elements beyond the first phalange. Including the minute discoidal terminal 

 elements, there are eight phalanges in the middle digit and seven in the first 

 and third. In this genus the hind limb (fig. 99) is relatively small. 



In contrast with the form of paddle seen in Mixosaurus that of Merriamia 

 is exceedingly narrow, and the podial portion is also relatively short. The 

 function of the paddle was evidently somewhat different from that in Mi.ro- 

 .^a unts. The relatively longer humerus and shorter podial region apparently 

 gave the limb somewhat more freedom of movement than in the case of the 

 limbs with shorter humerus and larger podial region as in Mixosaurus and 

 other broad-paddled ichthyosaurians. 



Toretocnemus. The anterior limb of Toretocnemus is unfortunately known 

 only in part. (fig. 139), but in as much of it as is present the structure resembles 

 that in Merriamia. Only digits one and two are preserved and in these the 

 terminal phalanges may be missing, but the fourth element of digit one and 



