LESSONS IN GEOLOGY. 



3S3 



undergone a peculiar bitnminisation, probably due to the ira- 

 a a nature of the clay in which it wu embedded. On 

 account of it* lightness and being niuoeptible of a very high 

 polish, it is inuoh used in making ornament*. The Whitby coal- 

 bed is a Hassle dopoiit. 



The fauna of the period is diversified and interesting. Many 

 of the beds being profusely fossiliferous, the limestone is some- 

 times called Qrypbite limestone, from the great number of 

 gryphroa it contains (Fig. 104). Of the ammonites upwards of 

 120 species have been discovered in the lias. These oophalapods 

 are so restricted to certain beds that, as we have said, the occur- 

 rence of a specimen frequently decides the position of the bed. 



At the base of the lower lias, for instance, two beds occur. 



The upper and larger is distinguished by the Ammonite* Buck- 



Fig. 105), while the A. planarbit (Fig. 106) characterises 



the lower one. These ammonites are found with every possible 



variety of external markings, from the smooth A. planorbit to 



rates are found of every sii, from half an inch in length to a 

 foot 



We have often enumerated the tyirijtrt amongst the fossa* 

 of the strata we have passed through ; but they do not seem to 

 have survived the liasaio period. Several species are found in 

 the lias, but none above it Many fish of the ganoid and the 

 plaooid tribe* appear. The JSckomodus (Fig. HO) is almost 

 exclusively liassio, and will five an idea of many of the flab of 

 the period. 



But by far the moat interesting feature in the life of the age 

 is the appearance of reptiles of extraordinary sue aad structure 

 A glance at the skeletons of the /eUAyoMwnu (Fig. Ill) aad 

 the PUtiowurui (Fig. 112), and the ideal sketch of the liaesic 

 forest where the restored creatures are in their native element. 

 will at onoe, without description, give an idea of the peculiarities 

 of the monsters. One specimen of the ichthyosaurus taken froso 

 the lias, and now in the British Museum, measures twenty-four 



IDEAL FLORA AND FAUNA OF THE LIA8SIC PERIOD. 



1. Ichthyosaurus ; 2, Flesiosaurus ; 3, Albertia ; 4, Cycadeae ; 5, Zainia. 



the rough and pointed A. margaritatus. Their ally, the nautilus, 

 differs mainly from them in the position of the siphnncle, which 

 is marginal in the ammonite, and central in the nautilus. Both 

 are curled shells, divided into chambers by septa, and where the 

 walls of the septa cut the shell externally, are the sutures. In 

 some coses these are plain lines; in others, crumpled. Fig. 107 

 is a specimen of Nautilus truncatus. The cephalopoda were 

 well represented in the liassic seas, for the belemnites are 

 doubtless the internal bone of a cephalopod, approaching the 

 cuttle-fish in structure. Figs. 108, 109 represents the fossil, and 

 its supposed position in the fish. They derive their name from 

 &f\tfi.vov, a javelin, for they have every appearance of pointed 

 stone bolts, which might be shot from a cross-bow, or used to 

 point a spear. They are of a fusiform shape, pointed at one 

 extremity, and having a conical cavity at the other, occupied by 

 a chambered shell, called the phragmacone. There is a small 

 central cavity, at the top of which is, or was, a thin horny case 

 containing the ink with which the animal could darken the 

 water and elude the search of its enemy. Specimens have been 

 found with the ink-bag in a fossilled condition, and the black 

 fluid dried into a hard substance, which, however, on the addi- 

 tion of a little water, assumed its original liquid state. Belem- 



feet; and the plesiosaurus was about the same size. The 

 peculiarly large eye, the formidable rows of teeth, the long 

 processes attached to the vertebra, bespeaking great muscular 

 power, all indicate that the ichthyosaurus must have been a 

 formidable foe, and probably the king of the waters. The 

 plesiosaurus is supposed to be more fitted, from its long and 

 comparatively more delicate structure, for the shallow water of 

 the shoals, where, propelled by immense paddles, it chased the 

 fish with a rapidity equal to their own. 



The remains of these reptiles are always found all altogether ; 

 the head, neck, and tail are usually in their proper position, 

 whilst the other bones of the skeleton are detached, and in a 

 confused heap. This is so constantly the case, that it demands 

 some explanation. That usually given is, that the reptiles 

 possessed a thick tough hide; when they died, they became 

 irflated with gas, owing to the decomposition of their entrails, 

 and floated on the water. Whilst in this state the bones of th? 

 skeleton became dislocated, but were held together by the akin ; 

 becoming in time water-logged, the carcase would sink to the 

 bottom, where it would be covered with the sediment then in 

 process of deposition, and thus the bones would be fossilised in 

 a heap. 



