198 ME. E. B. WJBTHERED ON THE [May 1895, 



other portions they are not so clearly defined — owing to the tubules 

 not having been cut through, and the massing together produces a 

 compact crust. 



My interpretation of the origin of this crust is that it has been 

 constructed by tubular forms of growth allied to Girvanella. The 

 transparent tubules are those which have been cut through in making 

 the preparation, thus exposing the infilling calcite in the interior. 



The dark portion represents tubules which have not been cut 

 through, and consequently the dark walls remain and when massed 

 together give tbe appearances represented in the less clearly defined 

 part of the crust. 



It is important to notice the irregular growth of the Girvanellce 

 in the nucleus of PI. VII. fig. 1. Some are seen in tangential, some 

 in vertical section. The same thing occurs in the crust. Some of 

 the tubules grow concentrically, others crop up among the latter 

 more or less at right angles, and are marked by transparent oval 



I have dwelt on the features exhibited in PI. VII. fig. 1 at some 

 length, because they have an important bearing on some of the 

 other granules to be referred to in the course of this paper. 



PI. VII. fig. 2. — This object may be taken to represent a well- 

 defined oolitic granule of the type usually figured in text-books, and, 

 looked at apart from the evidence drawn from other more clearly 

 defined granules, we should hesitate to affirm its organic origin. 



In the centre is a nucleus of some kind. This is immediately 

 surrounded by granular amorphous calcareous material and then by 

 concentric strata, the whole being finally encircled by a thin stratum 

 resembling somewhat the tubules which enclose the nucleus in 

 PI. VII. fig. 1. The other strata are well marked, and are for the 

 most part clear. They are, however, traversed by dark striae and 

 patches which radiate more or less at right angles to the nucleus. 



At the point C there is a break in the concentric arrangement, 

 and it is impossible to resist the conclusion that these appearances 

 are due to intersected tubules. But in some of the concentrically 

 formed granules the strata are masses of growth the outlines of 

 which have been obliterated, and this may be the case in the granule 

 before us. 



The next object (PI. VII. fig. 6 a) is representative of a common 

 form of granule of the type also represented by fig. 5. In one part 

 the crust surrounding the nucleus shows a clearly-defined concentric 

 structure, but this to a great extent disappears in the other portion. 

 The first question to be considered is whether this latter feature is 

 caused by obliteration of the original structure or from a difference 

 in the arrangement of the structure forming the crust. The second 

 question is whether the crust is of organic or inorganic origin. The 

 concentric structure is not suggestive of chemically-deposited layers 

 of calcium carbonate, but rather of organic growth, and if this were 

 not continuous in the same plane, then the fact would account for 



