26 INTRODUCTION. 



acute angle — usually only about 1 0° or 1 2° — with the surfaces of the shell. These prisms 

 range from about l-2000th to l-600th of an inch in breadth, the average being probably 

 about 1-1 000th of an inch, and are not above one tenth of the last-named amount in 

 thickness. With regard to their length, I am not able to speak positively, as I have never 

 succeeded in tracing any single prism continuously from one of its extremities to the 

 other ; but I have frequently met with prisms, broken at one end, whose length measured 

 l-50th of an inch. It sometimes happens that a shell of a recent Terebratula, which has 

 been kept for some time in spirit, can he readily resolved into such prisms, even by 

 rubbing portions of it between the fingers (PL I, fig. 5) ; but more commonly they are 

 best made apparent, by cleaving the shell in the plane in which they he. Most Brachio- 

 podous shells readily admit of this kind of cleavage ; but it is more easily practised with 

 the Rhi/nconellidcB, Spiriferidce, and Strophomenidm, than it is with the Terebratulidse. 

 When their natural laminae thus obtained are examined microscopically, the flattened 

 prisms are seen to lie side by side with great regularity (PI. IV, fig. 6 ; PI. V, figs. 4, 6), 

 usually overlapping one another at their edges; they are sometimes gently curved 

 laterally^ but they never seem to depart widely from the straight line in that direction. 

 The laminae which they thus form, however, are often far from being flat ; and this, which 

 is particularly the case in those parts of the shell whose curvature is great, is due to the 

 inflexion of the prisms in the direction of the thickness of the shell ; so that, when seen in 

 a vertical section, they often present considerable curvatures, especially towards their 

 extremities, when these abut upon the perforations for the vascular canals (PI. IV, fig. 4; 

 PI. V, fig. 14). The internal surface of the shell derives a most peculiar appearance from 

 the 'cropping-out' of these prisms at the acute angle just mentioned ; for it presents a very 

 regular imbricated aspect (PI. IV, figs. 7, 10, 11, 14; PI. V, figs. 4, 5), each of the 

 imbrications being the rounded extremity of one of the flattened prisms. The external 

 surface, owing probably to the incorporation of the periostracum with the terminations of 

 the prisms, does not distinctly show any such arrangement ; but, when a little abraded, 

 as happens in most fossil specimens, it very commonly exhibits the ends of the prisms 

 cropping-out upon the surface at an extremely slight inclination, but without any regularly- 

 formed terminations. When a section of the shell is made more or less parallel to its 

 surface, this may traverse the prisms for a time in their own plane, or nearly so, and 

 they will then present almost the same appearance as they do in a natural lamina ; but it 

 may cross the prisms more or less obliquely (PI. V, fig. 3), and the appearances presented 

 will vary according to the degree of that obhquity. Not unfrequently, owing to the 

 curvature which has been already noticed as presented by the prisms, a section will traverse 

 them in one part almost in the plane of their length, and will cross them very obliquely 

 at another. A great variety of appearances may thus be presented by the shells of 



^ That is, -when considered in relation to the shell of which they form part, their general line 

 of direction being from the umbo towards the margin. 



