MKURTAM: TRTASSTO ICHTHTOSATJEIA. 101 



six phalanges of each digit to show an elongated form with a median constric- 

 tioii. In the distal phalanges, which are possibly added elements, the notches 

 arc less distinct or absent. As originally suggested by Lydckker for the notches 

 on the margins of certain podial elements in the paddles of longipinnate 

 ichthyosanrs, these notches or constrictions evidently indicate an original 

 shafted form of the phalanges, which has been entirely lost in Baptanodon and 

 in most of the late ichthyosanrs. The constriction of the phalangeal elements 

 is in reality somewhat more strongly expressed on the specimens than it appears 

 in Repossi 's illustrations. 



On one specimen where both fore and hind limbs are represented the hind 

 limb appears to be less than two-thirds the size of the anterior-one, and the 

 1 tones of the pelvis appear correspondingly small. Both propodial and epipo- 

 dial elements are here more slender and generally more primitive than in the 

 anterior limb. (See fig. 97, p. 66.) The phalanges are also more slender and 

 seem not to have increased in number to such an extent. 



In the first row of the mesopodial section of the hind limb figured by 

 Repossi (1902, tav. 9, fig. 3), as in the anterior limb, four elements appear 

 instead of three. (See fig. 311, p. 70). The structure which appears in this 

 case is unusual but is probably explained by another specimen (fig. 110, p. 70) 

 in the Milan museum, in which only three elements are present in the prox- 

 imal row. The lunate intermediate element in the second specimen almost 

 closes the gap between the tibiale and fibulare, but in a second row of bones three 

 elements articulate with the distal border of the intermedium; of these the 

 anterior one has crowded up between the tibiale and intermedium so as almost 

 to corne in contact with the tibia. In the specimen figured by Repossi this 

 element of the second row seems to have reached the tibia. This explanation 

 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 apparently 

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

 This constricted bone has the appearance of a metapodial or phalange, and if 

 the explanation given above is correct it is really a metapodial moved upward 

 into the position of tarsale two. This does not, however, explain the presence of 

 lateral notches in other evident mesopodials shown in figure 110. 



Limb Arches. All four elements of the pectoral arch are figured by 

 Repossi. (See fig. 80, p. 59.) The coracoid and scapula are represented as 

 almost identical in form. So similar are they that unless corresponding 

 elements from the opposite side of the body are known, and show similar evi- 

 dence as to relative position, these two bones might be suspected of being 

 either two coracoids or two scapulae. Their form is not entirely similar to 

 that of any elements in the pectoral arches of other ichthyosaurians. 



