Structure of the Plesiosaurus Macrocephalus. 531 



The bones of the leg have the same distinguishing character as those of the fore-arm, and the 

 fibula in all the Plesiosaurs corresponds to the ulna in its peculiar bent figure. 



In tlie PL Macrocephalus the fibula is, however, relatively broader than in the PI. Hawkinsii, 

 notwithstanding that, like the ulna in the fore-arm its distal extremity is on the same plane with 

 that of the adjoining bone : it is, in fact, fully as broad as it is long, which proportions, com- 

 bined with the characters of the cervical vertebrae, distinguish it from the fibula of any other 

 species o( Plesiosaurus which I have yet seen. 



The tarsus consists, in PL Macrocephalus, of six instead of five bones, as in the PL Harv- 

 kins'ii ; it participates in the peculiarity of having three bones which are situated at the anterior, 

 or tibial side of the joint, much smaller than are those of the fibular side, and so placed, in the 

 intervening space between the tibia and tibia! metatarsals as to indicate that the foot had a freer 

 inflection forwards, or upon the tibia, than in the opposite direction. 



This structure. Dr. Buckland has observed to me, would give a compound motion to the pro- 

 pelling stroke of the paddle, similar to that which in skilful rowing is called feathering the oar. 



In the PL Hawkinsii the interspace between the tibia and metatarsals is occupied by a single 

 round and flat bone ; in the PL Macrocephalus by two ; the additional bone being situated at 

 this part of the tarsus. 



The metatarsals resemble, in number and disposition, those of the PL Hawkinsii, as is shown 

 in the following table : 



PI. Macroceph. Pi Hawks. 



The first, or tibial metatarsal supports .... '6 phalano-es 3 



The second 5 5 



The third 8 (or 9 ?) 8 



The fourth, more than 7 g 



The fifth 6 6 



In general form and proportions of the phalanges of both extremities, a close resemblance 

 subsists between the two species. 



Of the Head. (PI. XLV.) 



I have reserved for the conclusion of the present paper the description of 

 this, the most complicated and difficult part of the skeleton. The relative 

 size of the skull to the body and cervical portion of the spine in the Plesiosau- 

 rus Macrocephalus has already been stated ; but its structure and composition 

 still remain to be spoken of. From its position in the specimen, the composi- 

 tion of the upper and lateral parts only can be examined, but the bones on 

 these surfaces are well defined, and in tolerably perfect condition. 



The general form of the head is depressed and triangular as in the other 

 Plesiosaurs; but the breadth of the base of the triangle formed by the 

 posterior part of the head is relatively greater than in the PI. Hawkinsii. 



The elongated form of the strong and prominent cranial bones, most of 

 which are seen extending from point to point, with wide interspaces, like the 

 timbers of a scaffolding, indicates the affinity of the Plesiosaur to the Lacer- 

 tine Saurians, and this affinity will be further exemplified in the condition of 

 many of the individual bones. 



VOL. V. SECOND SERIES. 3 Z 



