i \ i i.i;\ M. 



i . explains tin- I'-SUSe "f large <|iianlilie- nl' -helU ..I 1 Spirilla 

 In-ill-^ ua-hed a-hore in h M-.-I I i I ie- ivmn\ed man\ hundred- <>l 

 mile- from t In- li:i Ii1 :ii of tin- :inini:il; il a l-o e \ phi i n- 

 1ori!\ t \\ o interest ini: paheontoh.-iea I facts, namel\. the in 



numerable quantity of fossil shells r<>un<l in i>ed- which repiv-ent 



ancient I'eaches.aiid t hci r a I selice I'rolll t IK >se 1 ic< I- \vh ich formed 



ottonis. 



Kxlernal cephalopodoue shells are ;ill -\ niiiict rir;i I except lln- 

 UTIHTM Tiirrilitrs jiml I K-licoccriis : tln-sr Inttci' iiistc;il of form 

 iiiii :i s|ir;il I'ollcil in tin- same plane, are obliquely spiral ; th;it 

 is. nn one siiU- i> tin- |)rojM-t inu sj>irr of the shell, on the other 

 tin- umlilie:il |K'iiin_n- >r :i\is of the volution.-. The syimm-l rienl 

 forms, very numerous. \;iry .-ill the \\-.\\- IVoni ;i st r.-ii^lit to ;) 

 eoilel o-ro \\tli. their ditl\-rence in pl:in of U,TO'.\ 1 h const it lit in LI t he 

 uenerie distinct ions ; l. In- enhiruvd upon in the descriptive por- 

 tion of this work. Il in:i\- le reninrked here, however, thai in 



some uvpeni :i chnnm- <>!' I'orni takes place after they h:i\e 

 attained a certain a;_M'- In Lituitrs the slu-ll commences with an 

 open spiral (with, disjointed whorls), and finally ^i-ows in a 

 straight line. In A nc\ loceras the commencement is similar, luit 

 after elongating the whorl for awhile the extremity is incurved. 

 And in Scaphites a similar mode of u;rowtli to Ancyloceras i^ 

 distinguished nevert heless l>ythe initial spiral whorls l.eiiiLi in 

 contact. All other moditical ions caused Iy Mii* 1 . do not chan-e 

 tin- form luit. merely the external surface of the shells. Some 

 to il Nautili, lor instance, striated when yoiin^. become smooth 

 at a later period, whilst others smooth when yoiin^ are striated 

 or rililted when adult. 



in the Ammonites, the juvenile shell is always smooth. 1ml 

 in Ihe course of urowt h, tubercles, rilis and slri;e a ppear. :ind 

 develop until the animal has become adult ; after this period 



iieration !ake> place, the oruament> -radually disappear in 

 the successive growths, and iii old age the surface of the outei- 

 whorl may lie as smooth as in youth. In Ammonite- of the 

 .-aim- -pecie- tw> lurms of shells may l>e o!i-er\ed. one much 

 compre-sed. the other swollen ; and it i- reasonable !> <-<,niect lire 

 fr<>m analogy with living species that the sexes are thus in- 

 dicated to II-. the swollen shell Itein^ required for t lie ovar\ of 

 the female. 



