Dr. H. A. Nicholson on the Oenus Strom atopora. 7 



openings in 8. concentrica ; and the evidence is not at present 

 sufficient to warrant any positive statement of opinion as to 

 whether they may correspond with the " pores " or the " os- 

 cula " of an ordinary sponge. It is probable, however, that 

 they should be regarded as representing the " pores," and tliat 

 the " oscula" will yet be discovered by a more extended and 

 complete examination. Again, in 8. ostiolata, Nich., a species 

 from the Guelph formation ot Canada, the upper surface of the 

 mass exhibits small but regularly arranged openings, which, 

 from their remoteness and general form, can hardly be regarded 

 as other than "oscula" ('Annals,' Aug. 1878, ph iv. fig. 1). 

 In S. tuherculata, Nich., again, I have now discovered a sys- 

 tem of comparatively large and remote openings, which com- 

 municate with canals traversing the organism, and which 

 appear to fulfil beyond all question the function of exhalant 

 apertures. In *S'. firanulata, Nich., no openings are ordinarily 

 to be detected, probably on account of the manner in which 

 specimens are generally preserved ; but I have one example 

 showing both small and large openings, which must be con- 

 sidered as being inhalant and exhalant. In 6'. perforata^ 

 Nich., now described for the first time, the entire mass, or 

 " sarcodeme," is traversed by numerous and close-set canals 

 of considerable size, which open at the surface in rounded 

 apertures generally ])laced on conical or chimney-like emi- 

 nences. These must represent exhalant canals and apertures. 

 Lastly, in 8. Hindci, also now described for the first time, 

 there is a series of small close-set apertures which must repre- 

 sent " pores," and another series of larger, more remote, and 

 more irregularly disposed openings which can only be regarded 

 as oscula. With the exception, however, of the last-named 

 species and of the single specimen of 8. gramdata above 

 alluded to, I am acquainted with no species of 8tromato2)ora 

 which has hitherto been shown with certainty to possess tico 

 sets of openings — one small and inhalant, the other large and 

 exhalant. It must be remembered in this connexion that the 

 difficulties in the way of observation are in this case extremely 

 great, since the condition of mineralization in which these 

 fossils occur is generally such that the cavities of the mass 

 are filled up with foreign material, whilst the reticulated tissue 

 itself is often silicified. Hence it would be easy for such 

 minute surface-apertures as the " pores " of a sponge to be 

 irrecognizably filled up and obliterated or to escape detection, 

 c. Tiie shape of the various species of 8tromatopora is such 

 as would acccjrd perfectly well with the belief that the genus 

 is referable to the S[)ongida. Some species arc in the form of 

 rounded or irregularly hemispheric or conical masses. Others 



