848 LECTURES ON GEOLOGY. 



termed bavin, whose thickness does not increase with that of the 

 stone. No satisfactory explanation has hitherto been given of this 

 form of stratification, nor why the fossils, though equally abundant 

 in every part, project in relief only from the upper surface of the 

 slab ; nor why the finest specimens are usually found in the 

 thickest and lowest strata. 



The Lecturer then exhibited specimens and drawings of the 

 more remarkable fossils of the limestone, and proceeded to point 

 out the peculiar structure of the ammonites and orthoceratites, and 

 other multilocular shells of this and the succeeding strata, by 

 means of which they are supposed to have had the power of rising 

 to the surface of the water, like the chambered nautilus, and little 

 spirula, the only animals of analogous form now existing ; for the 

 sepia that inhabits the paper nautilus is believed to be an intruder, 

 like the hermit crab, into the shell of some unfortunate mollusca it 

 has destroyed. The most remarkable fossil of the limestone is the 

 trilobite, whose place in the series of animated beings is very far 

 from being determined j some geologists referring it to the class 

 of insects, and others to the Crustacea, or crabs. It is of an oval 

 shape J like the crab it exhibits an almost plane surface, which 

 may be considered as its head or body, and attached to the body is 

 a three-lobed tail, each lobe of which consists of an equal number 

 of convex plates or rings, jointed to each other like those of the 

 tail of the crab or lobster. Like the Crustacea, its surface is 

 covered with minute tubercles and pores, but it differs from them 

 in the position of its eyes, if such they be, which are placed upon 

 the back instead of being beneath its projecting edge, as in that 

 class. As one species of trilobite is monocular, it is probable that 

 the tubercles, which have been taken for eyes, were destined to 

 some other purpose. Again, both the head or body, and the tail, 

 are surrounded by an elevated rounded margin, which is not the 

 case with the Crustacea. In this respect, as well as in its jointed 

 structure, it closely resembles the limpet chaeton ; and there is 

 reason to suppose the trilobite analogous to that shell, from the 

 circumstance that nothing but its oval crust has ever been disco- 

 vered J for were it a s})ecies of crab, the legs and claws, which are 

 of the same texture, and even harder than the shell of the body, 

 could not fail to have been discovered among the thousands of 

 specimens of different species of trilobites found in various parts 

 of the world. Mr. Parkinson compares the trilobite to an animal 

 which M. Latrobe has named oniscus pregustator, from its pecuhar 

 mode of life. At certain seasons the fish called the old wife 

 ascends York River, in the United States, from the sea, in vast 

 quantities, and each fish carries in its mouth an oniscus firmly 

 fixed there by its fourteen claws. Perhaps our doubts concerning 

 the nature of the trilobite may soon be dispelled, as the discovery of 

 a recent specimen in the Falkland Islands has been announced. 



Of all the petrifactions of the transition limestone, coral is the 

 most abundant, and in the greatest variety of forms. A mistaken 

 notion of the nature of coral has been long prevalent, that each 



