MINUTE STRUCTURE. 35 



The one feature which, perhaps moro conspicuously than any other, characterises 

 the entire group of tho Stromatoporoids, is the constitution of the skeleton, more or 

 less obviously, of superimposed concentric layers. Sometimes, as in Stromato- 

 pora conccntrica, Goldf., the skeleton is composed of concentric strata (" latilaminae) 

 of considerable thickness (Plate V, figs. 8 and 0). In such cases, the intervals 

 between two of the " latilaminae " merely mark periodic pauses in the growth of 

 the skeleton, and it is difficult or impossible to recognise any composition of the 

 individual strata out of secondary layers or " laminae." Each stratum, or 

 " latilamina," is made up of a scries of parallel vertical rods ("radial pillars"), 

 which run from the top to the bottom of the stratum, and are united at irregular 

 intervals by oblique or horizontal processes (Plate V, figs. 10 and 15 ; Plate XI, fig. 

 18). The intervals between these vertical rods are the tortuous tubular canals in 

 which the zooids of the colony were lodged, and they are often " tabulate." In 

 other forms the entire skeleton is made up of closely approximated concentric 

 layers, or " laminae," which may or may not be arranged in thick strata, or 

 " latilaminae " (Plate I, figs. 9 and 12). The " laminae " are not in absolute contact, 

 but are separated by narrower or wider interspaces (" interlaminar spaces "). These 

 interspaces are intersected by numerous vertical columns ("radial pillars"), which 

 connect together the laminae bounding the interspace on both sides, and may run 

 continuously through several interspaces and laminae. Reduced to its simplest 

 expression, the above may be taken as giving the essential structure of a typical 

 Stromatoporoid ; but it will be necessary to discuss the different elements of the 

 skeleton separately and in greater detail, and to consider the more important 

 variations which they exhibit in different types of the group. 



(a) The Skeletal Tissue. — The investigation of the ultimate structure of the 

 skeletal tissue of the Stromatoporoids is a matter of great difficulty, owing to the fact 

 that in many specimens the skeleton has undergone considerable secondary alteration, 

 while probably none retain their original constitution unchanged. There is, in fact, 

 considerable reason for concluding that the skeleton was originally composed of 

 arragonite, and that in almost all, or perhaps all, specimens which have not been 

 silicified, the arragonite has become more or less extensively replaced by calcite. 

 In the case of the Stromatoporoids from the pebbles of Devonian limestone con- 

 tained in the Triassic conglomerates of Devonshire, the skeleton usually consists, 

 like the matrix, of crystals of calcite, and is chiefly distinguishable from the matrix 

 by its darker colour. Hence, in these specimens little or no advantage can be 

 gained by the preparation of very thin sections, as the reduction of the slide to 

 extreme tenuity renders the skeleton more or less inconspicuous, or even undis- 

 tinguishable from the surrounding matrix. In specimens which have undergone 

 less alteration during the process of fossilisation, as in most of the examples from 

 the Wenlock Limestone of Britain, the skeleton of certain types {e. g. Actinostroma 



