ON BRITISH FOSSIL REPTILES. 157 



fined to the base or fang. The cavity of the tooth in Hypsodon would ap- 

 pear to have been much larger, as it is in many predatory iishes, in which the 

 teeth are more rapidly shed and renewed than in Crocodilian reptiles. 



The teeth of Poli/ptychodon differ from those supposed to have belonged 

 to Poiliilopleuron, in the ridges of the crown being more numerous and close 

 set, and in the transverse section being nearly circular instead of being ellip- 

 tical : from the teeth of Pliosaurus those of Polyptychodon differ in being 

 round and not three-sided, and in having longitudinal ridges over the whole 

 surface of the crown ; and from the teeth of 3Iosasaurus they differ in being 

 ridged and not smooth. 



Gigantic Fossil Saurian from the Lower Greensand at Hytlie. 



Under this head I have to notice some remains of a Saurian of marine 

 habits, but most probably of the Crocodilian order, as gigantic as the Ceiio- 

 saurus or Polyptychodon, but, in the absence of dental and vertebral charac- 

 ters, not referable to any known genus. These remains were discovered by 

 H. B. Mackeson, Esq. of Hythe, in the greensand quarries near that town, 

 and include portions of the iliac, ischial and pubic bones, a large proportion 

 of the shaft of a femur, parts of a tibia and fibula, and several metatarsal 

 bones. In consequence of the absence of vertebras and teeth, the present ob- 

 servations will be limited to indicating the characters by which these remains 

 differ from previously known extinct genera of Saurians. In the first place, 

 as the femur and other long bones have no medullary cavities, but a central 

 structure composed of coarse cancelli, it is evident that the animal of which 

 they formed part was of marine habits ; but the best- preserved bone being a 

 femur, this circumstance, independently of the size and shape of the metatar- 

 sals, at once negatives the idea that these remains belonged to the Cetacean 

 order, whilst the form and proportions of the metatarsals equally forbid their 

 reference to any other Mammalian genus. 



Femur. — The portions of this bone secured by Mr. Mackeson include 

 about the two distal thirds, excepting the articular extremity ; its length is 



2 feet 4 inches ; its circumference in the middle, or smallest part of the shaft, 

 is 15 inchec 6 lines, and at the broken distal end, 2 feet 5 inches. These 

 dimensions prove that the animal was equal to the most gigantic described 

 Iguanodon*. If the supposition of the proportion of the femur which has 

 been preserved be right, this bone differs from that of the Iguanodon, not 

 only in the want of a medullary cavity, but also in the absence of the compressed 

 process which projects from the inner side of the middle of the shaft. The 

 bone also expands more gradually than in the femur of the Iguanodon, and 

 the posterior jiart of the condyles nmst have been wider apart in conse- 

 quence of the posterior inter-condyloid longitudinal excavation being longer 

 and wider. 



Tibia and Fibula. — The portion of a tibia which has been preserved is 

 compressed near its head, and the side next to the fibula is slightly concave. 

 The longest transverse diameter is 8 inches 9 lines, and the two other trans- 

 verse diameters at right angles to the preceding, give respectively 3 inches 



3 lines, and 2 inches 6 lines. The bone soon assumes a thicker form, its cir- 

 cumference at about one-third from its proximal end being 16 inches G lines. 

 The cancelli occupying the central portion of the bone are arranged in a suc- 

 cession of layers around a point nearest tlie narrower end of tlie transverse 

 section. Lower down the tibia again becomes compi*essed, and towards the 



* The length of the largest femur yet obtained of this Saurian is 4 feet 6 inches, its 

 smallest circumference I foot 10 inches. 



