62 



GEOLOGY. 



measured 7 feet in length ; its jaw was 6 feet long, and one of its 

 teeth was 15 inches in length. It had a shorter neck than the 

 Plesiosaurus, hut was probably less fish-like in aspect than Ichthyo- 

 saurus, which latter reptile it outrivalled in point of size. 



In Wall-case 11, and in Table-case No. 15, are arranged the extinct 

 group of marine reptiles, the Plesiosauria. (See Woodcut, Fig. 29.) 

 They are distinguished at once by the great development of the neck, 

 which is composed of numerous vertebrae. The head is compara- 

 tively small in size ; the orbits were large ; the limbs were shaped 

 externally like the flippers of a whale, and made up of 5 fingers, com- 

 posed of numerous phalanges. The jaws were armed with many simple 

 pointed teeth inserted in distinct sockets. The most complete examples 

 are the Plesiosaurus Hawkinsii, the PI. robustus, the PI. laticeps, 

 all in Case 11; and the cast of the great PL Cramptoni (on the wall 

 of the passage leading to the S.E. gallery), which is 22' 0" in length 

 and 14/ 0" in breadth, measuring across its expanded paddles. 



Most of these old Marine-Lizards, both the long and the short- 

 necked forms, were obtained from the Lias of Street, Somersetshire, 

 Lyme Kegis, Dorsetshire, Barrow-on-Soar, Leicestershire, and 

 Whitby, in Yorkshire. 



Wall-case 12, and Table-cases Nos. 3 3 and 14, Ichthyosauria, 

 *' Fish-Lizards." — These great marine reptiles had very short necks 

 (see Woodcut, Fig. 30), probably not visible at all externally; the 

 vertebrae were numerous and deeply biconcave; the skull had very 

 large .orbits, and the eyes were surrounded by a ring of broad bony 

 (sclerotic) plates. The jaws were elongated, and armed with powerful 

 teeth implanted in distinct sockets. The fore and hind limbs were 

 converted into fin-like organs, composed of short polygonal bones, 

 arranged in five closely approximated rows, with supernumerary rows 

 of margiual ossicles added. 



The largest entire Ichthyosaurus is from Lyme Regis, and measures 

 22 feet in length and 8 feet across the expanded paddles ; but detached 

 heads and parts of skeletons prove that they often attained a far 

 larger size than this. 



In some of the Ichthyosaurs the jaws are prolonged into a long and 

 slender rostrum ; others have short and robust heads, and jaws armed 

 with large teeth. 



These old marine lizards must have exercised the same repressive 

 action over the teeming animal population of the old Liassic seas that 

 the sharks do in our seas at the present day. Their distribution is 

 similar to that of Plesiosaurus. 



In Wall-case No. 9, is arranged a further series of S. African 

 reptilia belonging to the division Anomodontta (Owen), such as Dicy- 

 no don, &c. 



CLASS 4. — AMPHIBIA. 



In Wall-case No. 8, and in Table-cases Nos. 21 and 22, are placed 

 the fossil Amphibia (Frogs, Toads, Newts, and Salamanders). — These 



