Page:Geology and Mineralogy considered with reference to Natural Theology, 1837, volume 1.djvu/267

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The shell of the Ammonites Heterophyllus (Pl. 38, and Pl. 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 revolution of the shell; in this, as in almost all other cases, the extreme beauty and elegance of the foliations result from the repetition, at regular 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 Pl. 38, c.)

The figures of A. obtusus, (Pl. 35 and Pl. 36,) show the relations between the external shell and the internal transverse partitions of an Ammonite. Pl. 85 represents the form of the external shell, wherein the body occupied the space extending from a. to c., and corresponding with the same letters in Pl. E. 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. (Pl. 35, e.) A small portion of the shell is also preserved at Pl. 35, b.

From d. inwards, these sinuous lines mark the terminations of the transverse plates at their junction with the external shell; they are not coincident with the direction of the ribs, and therefore more effectually cooperate 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 surface.