530 Richard Owen, Esq., on the 



The carpus consists, in PI. Macrocephalus, of eight, instead of, as in PL Hawkinsii, six ossicles. 

 The fourth, or additional one in the first or proximal row, is wedged in between the ulna and the 

 third carpal bone, at the outer angle of the carpal joint; it is much smaller than the rest. 



The relative sizes, also, of the three normal bones of the first row is different. In the PI. 

 Hawkinsii the middle one is the largest ; the radial, or anterior one, the least. In the PI. Macro- 

 cephalus the ulnar, or posterior of the three, the largest. 



The disproportionate size of the two posterior bones in thePZ. Hawkinsii compensates for the 

 shortness of the ulna. In the distal row of the carpus the superadded bone in the PI. Macro- 

 cephalus is a very small ossicle wedged in between the third or posterior carpal, and the fifth or 

 ulnar metacarpal bones. 



The metacarpal bones correspond with those of the PI. Hawkinsii ; the radial or anterior one 

 which represents the pollex being the shortest and broadest. 



PI. Macroceph. PI. Hawks. 



The first metacarpal supports .... 2 (but probably 3) phalanges . . 3 



The second metacarpal 6 6 or 7. 



The third ditto 9 8 or 9. 



The fourth ditto 8 , 8 



The fifth ditto 6 6* 



The evidently natural curve formed by the distal phalanges in Lord Cole's PI. Macrocephalus 

 indicates that the paddles had a greater flexibility of this tapering extremity than those of the 

 Cetacea possess. 



Pelvic Extremitt/. 



Of the bones composing the pelvic arch, the outer convex margins of the two pubic bones, 

 small portions of the two ischia, and the femoral extremity of the left ilium are visible. They 

 show that the general form and relative proportions of these bones correspond with those of the 

 better known species of Plesiosaurus, 



There is, however, an evident difference in the proportion of the length of the pubis to that of 

 the vertebral column. We have enough of the pubis to ascertain that its length somewhat ex- 

 ceeded the length of four of the lumbar vertebrae ; while in PI. Hawkinsii, the length, or antero- 

 posterior diameter of the pubis is only equal to three and ahalf of the lumbar vertebrae : and this 

 difference evidently does not altogether depend on a smaller antero-posterior diameter of the ver- 

 tebrae in the PL Macrocephalus, but in a greater development of the pubis, which is in relation 

 to the greater size of the hinder paddle in this species. 



The femur in the Plesiosaurus Macrocephalus is relatively longer than in PI. Hawkinsii. In 

 the latter it equals the humerus in length ; in the former it exceeds the same bone by one-eighth 

 of its own length. 



In the PL Macrocephalus it is rather more expanded at the distal extremity than in the PL 

 Hawkinsii ; but the difference of form is not so well marked as in the humeri of these two 

 species. 



* In the enumeration of the Phalangeal bones by Mr. Cony beare (Geological Transactions, i. 

 p. 387.) the metacarpals are included ; the perfect digits of the PL Dolichodeirus correspond in 

 number with PL Hawkinsii and PL Macrocephalus. 



