302 



CORAL BAG. 



[Ch. XX. 



Fig. 352. 



Trigonellites latus. 

 Kimmeridge clay. 



I have seen them, on decomposing, leave the surface of every ploughed 

 field literally strewed over with this fossil oyster. The 

 Trigonellites latus (Aptychus, of some authors) (fig. 

 352) is also widely dispersed through this clay. The 

 real nature of the shell, of which there are many spe- 

 cies in oolitic rocks, is still a matter of conjecture. Some 

 are of opinion that the two plates formed the gizzard of 

 a cephalopod ; for the living Nautilus has a gizzard with 

 horny folds, and the Bulla is well known to possess one formed of calca- 

 reous plates. 



The celebrated lithographic stone of Solenhofen, in Bavaria, belongs 

 to one of the upper divisions of the oolite, and affords a remarkable ex- 

 ample of the variety of fossils which may be preserved under favorable 

 circumstances, and what delicate impressions of the tender parts of cer- 

 tain animals and plants may be retained where 

 the sediment is of extreme fineness. Although 

 the number of testacea in this slate is small, and 

 the plants few, and those all marine, Count 

 Miinster had determined no less than 237 spe- 

 cies of fossils when I saw his collection in 1833 ; 

 and among them no less than seven species of 

 flying lizards, or pterodactyls (see fig. 353), six 

 saurians, three tortoises, sixty species of fish, 

 forty-six of Crustacea, and twenty-six of insects. 

 These insects, among which is a libellula, or 

 dragon-fly, must have been blown out to sea, 

 probably from the same land to which the flying 

 lizards, and other contemporaneous reptiles, re- 

 sorted. 



Pig. 353. 



Skeleton of Pterodactylus 

 crassirostris. 

 Oolite of Pappenheim, near So- 

 lenhofen. 



MIDDLE OOLITE. 



Coral Rag. — One of the limestones of the Middle Oolite has been 

 called the " Coral Bag," because it consists, in part, of continuous beds 

 of petrified corals, for the most part retaining the position in which they 

 grew at the bottom of the sea. In their forms, they more frequently 

 resemble the reef-building poliparia of the Pacific than do the corals of 

 any other member of the Oolite. They belong chiefly to the genera 

 Tkecosmilia (fig. 354), Protoseris, and Thamnastrma, and sometimes 

 form masses of coral 15 feet thick. In the annexed figure of a Tham- 

 nastrosa (fig. 355), from this formation, it will be seen that the cup- 

 shaped cavities are deepest on the right-hand side, and that they grow 

 more and more shallow, until those on the left side are nearly filled up. 

 The last-mentioned stars are supposed to represent a perfected condition, 

 and the others an immature state. These coralline strata extend through 

 the calcareous hills of the N. W. of Berkshire,, and north of Wilts, and 



