MIXUTE STRUCTURE OF STROMATOPORA AND ITS ALLIES. 205 



falling short of the horizontal lamina above that from which they 

 spring. In such cases they appear in weathered specimens as so 

 many elevated tubercles on the surfaces of the laminae. In some 

 cases, whicli will be afterwards alluded to, they are replaced by 

 downward inflexions of the horizontal laminse themselves ; and 

 in some aberrant forms, which will be noticed hereafter, they are 

 entirely absent. Though they may be accidentally superimposed 

 upon one another in successive interlaminar spaces, they are 

 clearly independent of one another in origin, and are severally 

 confined to their own interspaces. Even, therefore, if they were 

 perforated centrally, they could not be compared properly with 

 the tubes of Tubipora or Fistiilipora. 



8. Tlie Structure of the Horizontal Lamince. — The horizontal 

 laminae which form the greater part of the mass of a typical Stro- 

 matoporoid appear in vertical sections as so many continuous con- 

 centric layers, whilst in horizontal sections they are necessarily 

 only visible where their cut edges may be brought into view in 

 consequence of their undulations. They can therefore be only 

 to a certain extent studied by means of thin slices prepared for 

 the microscope, since vertical sections simply cut them trans- 

 versely and leave many points unelucidated ; whilst horizontal 

 sections cannot be made to show more than the most minute 

 portions of the surface in the plane of any single lamina. "We 

 have therefore to rely principally upon the appearances presented 

 by the horizontal laminae as exposed on the surface of unaltered 

 calcareous specimens, or, still better, as seen in weathered or 

 decalcified specimens in which the skeleton has been silicified. 

 Even when examined, however, both by sections or by weathered 

 specimens, even when beautifully preserved, the horizontal 

 laminae do not present such clear or such uniform appearances 

 that a positive opinion as to their minute structure can be rashly 

 formed. On the contrary, it would appear that the actual struc- 

 ture of the horizontal laminae is diff'erent in diflferent Stromato- 

 poroids. 



In a great many Stromatoporoids (such as Stromatopora stria- 

 tella, D'Orb., S. granulata, Nich., S. tulerculata, Nich., &c.) the 

 surfaces of the horizontal laminae as exposed on weathered 

 surfaces are seen to be studded with elevated granules or 

 tubercles, sometimes extremely minute, sometimes large, papilli- 

 form, and prominent. These elevations are sometimes simply 

 rounded; at other times they are confluent, and form vermiculate 



16* 



