VARIED PROPORTIONS OF SUPPORT. 263 



The shell of the Ammonites Heterophyllus (PI. 38, and 

 PI. 39,) affords beautiful exemplifications of the manner in 

 which the mechanical strength of each transverse plate is so 

 disposed, as to vary its support in proportion to the different 

 degrees of necessity that exist for it in different parts of the 

 same shell.* 



complicated, the number of transverse plates is but sixteen in one revo, 

 lution of the shell ; in this, as in almost all other cases, the extreme 

 beauty and elegance of the foliations result from the repetition, at regu- 

 lar intervals, of one symmetrical system of forms, viz. those presented 

 by the external margin of a single transverse plate. No trace of these 

 foliations is seen on the outer surface of the external shell. (See PI. 

 38, c.) 



The figures of A. oblusus, (PI. 35 and PI. 36,) show the relations between 

 the external shell and the internal transverse partitions of an Ammonite. 

 PI. 35 represents the form of the external shell, wherein the body occupied 

 the space extending from b. to c, and corresponding with the same letters 

 in PI. 36. 



This species has a single series of strong ribs passing obliquely across the 

 shell of the outer chamber, and also across the air-chambers. From c. to the 

 inmost extremity of the shell, these ribs intersect, and rest on the sinuous 

 edges of the transverse plates which form the air-chambers. These edges 

 are not seen where the outer shell is not removed* (PI. 35, e.) A small 

 portion of the shell is also preserved at PI. 35, b. 



From d. inwards, these sinuous lines mark the terminations of the trans- 

 verse plates at their junction with the external shell ; they are not coincident 

 with the direction of the ribs, and therefore more effectually co-operate with 

 them in adding strength to the shell, by affording it the support of a series 

 of various props and buttresses, set nearly at right angles to its internal sur- 

 face. 



* Thus on the back or keel, PI. 39, from V. to B., where the shell is nar- 

 row, and the strength of its arch greatest, the intervals between the septa 

 are also greatest, and their sinuosities comparatively distant; but as soon as 

 tlie flattened sides of the same shell, PI. 38, assume a form that offers less 

 resistance to external pressure, the foliations at the edges of the transverse 

 plates approximate more closely ; as in the flatter forms of a Gothic roof, the 

 ribs are more numerous, and the tracery more complex, than in the stronger 

 and more simple forms of the pointed arch. 



The same principle of multiplying and extending the ramifications of 

 the edges of the transverse plates, is applied to other species of Ammo- 

 nites, in which the sides are flat, and require a similar increase of sup- 

 port ; whilst in those species to which the more circular form of the sides 



