138 PICTURESQUE SKETCHES 



what was once the under surface of decayed wood 

 drifting through the water. 



Several varieties of star-fishes, and some curious 

 forms not unlike certain recent crustaceans, were 

 among the common tenants of the lias ; and they 

 were accompanied by a large number of animals 

 inhabiting shells of various kinds, most of them 

 very different from those known at present. Among 

 these are both bivalves and univalves, the former 

 including a good number of the Brachiopoda al- 

 ready referred to, and belonging to groups, such as 

 Terebratula, still represented. The univalves, besides 

 a considerable number having near relations with 

 those of existing seas, include also a very large and 

 important group, highly characteristic of the secondary 

 period, and now absolutely extinct. I allude to the 

 so-called Ammonites, the nearest analogue of which 

 is the Nautilus, an animal most of whose peculiari- 

 ties of structure are now known, although much has 

 still to be learnt with regard to its habits. This 

 animal (the nautilus) is almost our only guide in 

 working out the various interesting points con- 

 nected with the extinct but nearly allied group of 

 which the ammonite is in some respects the most 

 perfect type. 



Reference has already been made, in speaking of 

 the extinct forms of an earlier period, to the peculiar 

 groups of cephalopodous animals whose remains are 

 met with in the older rocks. Of these animals there 

 are two \vell-marked groups represented at the pre- 

 sent day, the one by the nautilus, and the other by 

 the cuttle-fish. Of these, the former (see fig. 53) in- 

 habits a univalve shell, divided into compartments by 



