532 MM. Nicholson and R. Etheridgp, jun. — 



pi'esent — the size of tlie component tubes apart — the closest possible 

 resemblance to corresponding sections of Ghcetetes, while others 

 exhibit a strong likeness to Tetradinm. The extraordinarily minute 

 size of the tubes of Sohnopora would, however, certainly render it 

 improbable that we had to deal with a species of Chcetetes or Tetra- 

 dinm. Indeed, upon this ground alone, it might be considered as 

 seriously doubtful if Sohnopora could be referred to the Actinozoa at 

 all. If it be Coelenterate, it would perhaps more properly find 

 a place among the Hydrozoa rather than the Actinozoa. We are not, 

 however, acquainted with any Hydrozoon, living or extinct, with 

 which Sohnopora could be compared. It shows no features in its 

 minute structure which remind us of the Hydrocorallines, and it 

 assuredly presents no structural resemblance to any known type of 

 the Stromatoporoidea. 



The only other direction in which one might look for the true 

 place of Sohnopora is among the calcareous Algae. In its external 

 appearance, in its general texture, and in the aspect of fractured 

 surfaces, it presents a remarkable similarity to the massive forms of 

 the Nullipores [Lithothamnion) . In all the Nullipores, however, as 

 in the Algge generally, the minute structure of the organism is 

 essentially cellular and not tubular. In none do we find the long 

 capillary tubes, with their transverse tabulse and fissiparous mode 

 of growth, such as are characteristic of Sohnopora. Moreover, if 

 an argument against the reference of Sohnopora to the Coelenterata 

 be founded upon the fact that its tubes are much more minute than 

 those of any known Actinozoon or Hydrozoon, the same argument 

 may be reasonably employed, in an opposite direction, as against a 

 reference of Sohnopora to the calcareous Algge. In other words, the 

 miniite structure of Sohnojyora is as much grosser than that of the 

 Nullipores, as it is finer than that of the Coelenterates generally. 

 This will be seen by a reference to PI. XIII. Figs. 10 and 11, which 

 repi'esent sections of a living Nullipore on the same scale of enlarge- 

 ment as the Figures 4 and 5 of Sohnopora. This difference may be 

 roughly put in this way, that while a two-inch objective is sufficient 

 for a general examination under the microscope of a thin section of 

 any species of Chcetetes or Tetradium, one requires a one-inch 

 objective in order to clearly make out the structure of a similar 

 section of Sohnopora, and one cannot recognize the characteristic 

 structure of a section of a Nullipore with a lower power than a 

 quarter-inch objective. 



In deciding, however, as to any possible relationship between 

 Sohnopora and the calcareous AlgaB, it is necessary to consider more 

 minutely certain points in the structure of this curious fossil. Most 

 examples of Sohnopora compacta show concentric lines of growth, 

 which differ in no essential particular from those of many " Tabulate 

 Corals" {e.g. species of Monotrypa, Stenopora, Chcetetes, etc.). That 

 is to say, their growth was interrupted by periodic pauses, signalized 

 commonly (though not necessarily) by a simultaneous development 

 of tabulas, the same tubes subsequently continuing their upward 

 growth (PI. XIIL Fig. 4). 



