No. 136.] 



43 



short ; some have a narrowed or contracted portion of the outer 

 chamber, others are swelling or pear-shaped. Other species of 

 allied genera are curved into hooked or crescent-shaped forms ; 

 others still into circles or discs, thus approaching the form of 

 Nautilus. The partitions of the chambers also vary; some hav- 

 ing only a simple curve, others a waved or sinuous form : in some, 

 as in the discoid genus called Goniatites, the edges of the par- 

 titions have an indented or zigzag outline. In rocks newer than 

 the coal, forms of straight and coiled shells allied to Orthoceras 

 and Nautilus {^^ Baculites and Ammonites^^) are very abundant, 

 but in them the edges of the interior partitions are curved and 

 plicated into every degree of intricacy ; and looking at the whole 

 series of variations, it would seem as if there had been a purpose 

 to show every possible change in the arrangement of one general 

 plan or type. The siphuncle, or tube connecting the chambers, is 

 sometimes central, and sometimes lies against the inner or outer 

 wall of the shell. 



Of all these many varieties of one leading form, the straight 

 Orthocerata with simple partitions characterize the Palseozoic 

 rocks, as few or none are found above the Coal. The Goniatites, 

 with zigzag partitions, are found in the upper palseozoic rocks and 

 in some higher strata ; the Ammonites, and other forms with pli- 

 cated partitions, are found only in rocks between the Carbonife- 

 rous and Tertiaiy. The simpler coiled forms, like Nautilus, are 

 found in rocks of almost all periods. Less than half a dozen 

 existing species of chambered shells are now known, all of them 

 inhabiting tropical seas ; but the extinct species of the rocks are 

 numbered by thousands, and are found in every latitude (one of 

 many facts which seem to prove that the temperate and arctic 

 zones were once far warmer than they are now). Some of the 

 Orthocerata are no larger than a pencil, while others are found 

 in the Birdseye and Black-river limestones ten feet in length ; one 

 of which size lies in the Curator's room. Ammonites are also 

 found in the English chalk rock, as large as a wagon-wheel. 



The nature of these shells has been explained thus at length, 

 as they are among the most abundant and conspicuous fossils of 

 all the rocks of New York. Fig. 5 represents an Orthoceras from 

 the Birdseye limestone : others are figured in woodcuts illustra- 

 ting fossils of the higher rocks. The Black-river limestone con- 

 tains, as has been said, a large variety of these shells, not only 

 of the straight Orthocerata, but one or two coiled forms. Speci- 



