of the Chalk. 95 



regular octahedron is formed round about the point of union of 

 every four squares throughout the whole body of the Ventri- 

 culite, thus forming, of course, as many octahedrons as there 

 are squares. See Plate VII. fig. 8, and more clearly fig. 10, as 

 seen in flint, and fig. 9 as seen in chalk*. 



It is needless to point out the strength given to the whole 

 membrane by this arrangement, and the obvious design of it to 

 prevent any injury of the animals to which it belonged arising 

 from any yielding or distortion to which it might otherwise be 

 liable from the operation of ordinary causes present in the ocean 

 where they dwelt. They generally suffered fracture rather than 

 yield to such impulse. 



As the texture reaches the surface, both external and internal, 

 it assumes a different appearance in order to attach to it the 

 more securely what I shall crave permission to call, for the pre- 

 sent, the polyp-skin. The regular squares and their octahedral 

 junctions are still present, but under a different aspect ; their size 

 being contracted to about the 300th of an inch. This appears 

 to be effected by the addition of finer fibres crossing each other 

 within each square, so as to make at least four squares equivalent 

 to each of the larger ones. This membrane presents a solid 

 series of these squares ; that is, they extend, in a single layer, as 

 well in the plane of its thickness as of its superficies. The 

 membrane spreads over every convolution and descends every 

 anfractuosity of the body. Plate VII. fig. 1 1 shows this membrane 

 in calcedony, where several of the actual fibres are preserved and 

 the crystallization round them is very visible. 



External to the whole, the polyp-skin itself, — spread over 

 both external and internal surface and depressed also into each 

 of the anfractuosities, — stretches over the delicate membrane last 

 described, with which it is closely united (see Plate VII. figs. 12 

 and 13, and PI. VIII. fig. 6). In specimens both in flint and chalk, 

 prepared with care and in a high state of preservation, an equi- 

 distant row of apparent denticles seems to extend from the inner 

 substance to the external encasing wall. This is caused by the 

 transverse fibres (in the flint, incrusted with calcedony ; in the 



* It is obviously impossible to find individual specimens which shall 

 fully show those states of fact which have been only ascertained by careful 

 examination of many hundred specimens. Again, the least obliquity in 

 any section will cause an apparent elongation of some and a cutting off of 

 parts of other squares ; while the slightest variation in focus will cause fibres 

 in different planes to appear on the same plane. Hence, oftentimes, an ap- 

 parent irregularity which does not really exist, but which may easily deceive 

 an unpractised eye either in the object itself or in the engraving. Figs. 8, 11, 

 13 and 14 illustrate this. It should be added, that the octahedral structure 

 could not be given with any clearness in figs. 8, 9 and 14, without some- 

 what exaggerating its relative size. No engraving can do justice to the 

 exquisite delicacy of the original. 



