Observations on the Structure of Mesosaurus. 105 



preserved ; and on the left side a small part of the displaced ilium 

 is seen. The pubis and ischium are flat bones which bear consider- 

 able resemblance to those of Stereosternum. The pubis of Mesosaurus 

 differs from that of Stereosternum in that the pubic foramen is here 

 closed externally by cartilage and not by bone, as the American 

 genus. The ischium differs in having the external border less 

 concave than in Stereosternilm. The ilium is too imperfect to 

 admit of description. 



Each femur is well preserved, and is present as a slightly curved 

 rod of bone with each end abruptly truncated. It is thus evident 

 that each end of the bone was largely cartilaginous in life. The 

 inner end is slightly thicker than the outer. 



The cast of the left tibia is nearly perfect, and shows the bone to 

 have been an almost straight rod, slightly constricted in the middle 

 and a little thicker at the upper than the lower end. 



In the case of both fibulae the casts are imperfect at the 

 proximal ends. The fibula appears to have been rather stouter 

 than the tibia, and slightly curved — the concavity being directed 

 towards the tibia. 



The tarsus is represented by fairly satisfactory casts on each side. 

 It consists of two large bones in the proximal row and four in the 

 distal. Of the two proximal bones, the larger is probably the anchy- 

 losed intermedium and tibiale, and the smaller the fibulare. Between 

 the two bones there is a fairly large foramen. The first, second, and 

 third tarsalia are well preserved, and are small rounded bones of 

 about equal size. The fourth is not very well preserved, but appears 

 to be similar to the others. I can find no trace of a fifth tarsale. 

 Possibly it existed as a cartilaginous element. 



The impressions of all the digits are practically perfect. The first 

 has a rather stout metatarsal and two short phalanges — the two 

 phalanges being together shorter than the metatarsal. The terminal 

 phalanx does not appear to have been clawed, though the evidence 

 on this point is not quite conclusive. The second digit is consider- 

 ably longer than the first. ,The metatarsal is narrower and of more 

 uniform thickness. There are three phalanges which together are 

 about equal in length to the metatarsal. In the third digit the 

 metatarsal and first phalanx are each distinctly longer than in 

 the second toe. There are four phalanges. The fourth and fifth 

 toes are about equal in length and each considerably longer than the 

 third toe. In the fourth toe there are five phalanges, each of which 

 is, like the phalanges of the first three toes, moderately robust. In 

 the fifth toe there are four phalanges, which are each distinctly more 

 slender than those of the other toes. 



