42 [Assembly 



dijSerent species of Orthoceras, and other shells of similar char- 

 acter. This family of shells is a very peculiar one, found only in 

 the older or palsBozoic rocks, and thus are characteristic fossils 

 of the earlier periods of geologic history. They are, and have 

 long been, entirely extinct, and their structure is only understood 

 by comparing them with the Nautilus* now found living in the 

 seas of warm climates. The JYautilus is a coiled shell, whieh, 

 instead of being open within in one long spiral cavity like a 

 snail-shell, is divided into many chambers by a great number of 

 shelly partitions. These are formed by the inhabiting animal, 

 which is a mollusc of high organization, much like the cuttlefish 

 or squid. As it increases in size, and requires larger room for its 

 accommodation, it constantly adds to its shell at the enlarging 

 mouth, and cuts off the inner spaces from which it withdraws by 

 walls of shell. Thus in time, a long series of chambers, scores 

 in number, is formed, each larger than its predecessor as we 

 examine them in order from the centre through the enlarging 

 coil. The whole series are perforated by small apertures near 

 the centres of the partitions, through which there passes a long 

 membranous pipe or tube (known as the " siphuncle "), extending 

 from the animal living in the outer chamber to the innermost 

 apartment at the centre of the coil. It is believed that this 

 series of empty chambers acts as a buoy, by which the tenant is 

 enabled to float at will at the surface of the sea. 



(A recent shell of a Nautilus of the umbilicated species, nearly 

 allied to the Pearly Nautilus, lies in a case with some fossil shells 

 of similar nature in a window on the north side of the Hall ; one 

 of its sides being partly removed, to allow the internal structure 

 to be plainly seen.) 



The Orthoceras resembles the Nautilus, except that instead of 

 being coiled up in a circular form or disc, the shell is extended 

 in a straight line, forming a long and tapering cone. In many 

 specimens, when broken, the separate chambers and the mark of 

 the connecting siphuncle are plainly seen. They are very 

 various ; some being plain, or marked only with fine lines either 

 running circularly round the shell, or crossing each other at right 

 angles ; others being covered, as if for ornament, with protube- 

 rant knobs or rings : some are very long and slender, others are 



* The Pearly Nautilus is the shell spoken of: the thin "Paper Nautilus " is an entirely 

 different form. 



