54 Transactions of the 



The sclerotic coat, or capsule of the eye, which is bony, is also 

 found sometimes. 



The tail is the chief instrument of progression in these animals, 

 and assumes two chief modifications. In the greater number of 

 existing species, the tail is homocercal, or with two equal lobes, 

 and so it is with the fishes of the upper formations. But with 

 the magnesian limestone, we meet heterocercal, or fishes with 

 unequally lobed tails, the backbone running out into one side 

 alone, while the other is hardly developed. 



But some of the best fossils to be obtained from Aust Clifi" are 

 the Ichthyrdorulites, or the first rays of the dorsal fins. Many 

 extant species possess such spines, as the sharks, the rays, and 

 common dog-fish. They are fastened in the flesh, and are attached 

 to muscles, by which they can be raised or depressed ; they are of 

 a fibrous, osseous texture. The fossil spines occur in all the sedi- 

 mentary deposits, peculiar forms appearing in the several forma- 

 tions. They were probably used as weapons of defence. 



Among the fish of which the teeth are found at Aust is the Cera- 

 todus emarginatus. These present a very curious appearance, and 

 are found in the oolite as well. Their structure, in the crown of the 

 tooth, exhibits minute vertical pores, like the Psammodus poro- 

 sus : they consist of consolidated plates, instead of separate teeth ; 

 there was probably but one plate on each side of the jaws. The 

 upper margin is generally undulated, apparently from use. The 

 dental plates are composed of two distinct layers ; the root, of an 

 osseous reticulated tissue ; the crown, of dentine. 



Among the most extraordinary and rare relics to be found at 

 Aust are the elytra of insects. These elytra are composed of the 

 substance, called " chitine," which forms the skeleton, as it were, 

 of insects. The front pair of wings in the coleoptera are of chi- 

 tine, and protect the wings from injury, as the beetles are bur- 

 rowers. Other insects are found elsewhere, chiefly libellulidse ; 

 some wings of hymenoptera have been discovered, and a few dip- 

 tera ; but from the hard and enduring nature of the segments and 

 extremities of coleoptera, they are most frequently found. The 

 formations which have yielded most insects are the carboniferous 

 limestone, Jurassic, especially the lithographic limestones of Solen- 

 hofen, in Germany, and the lias, and occasionally the weald, but 

 these remains are not so common as might have been expected. 

 The tertiary marls of the Continent, as at Aix, are, however, found 

 to yield, among other curious fossils, some spiders and other insects. 



In the lias of Wainlode Cliff, on the Severn, some specimens of 

 minute neuropterous wings have been discovered, resembling the 

 recent Panorpa, especially P. gernianica, and have been termed 

 P. lia&sica. 



