MTNIITE STEUCTURE OF STT^OISFATOPOE A AND ITS ALLIES. 23B 



number of deep cups, wliich (on the view that PaJcsacis is a 

 Coral) have beeu generally regarded as "calices." In their in- 

 ternal structure all the specimens of Palceacis are shown by 

 microscopic examination to be composed of a reticulated calca- 

 reous tissue, which presents a close general resemblance to that 

 of certain of the Stromatoporoids, while the surface is covered 

 with tubercles and vermiculate ridges very similar to those ex- 

 hibited by such forms as Stromatopora tuber cidata, Nich., ^S'. dis- 

 coidea, Lonsd., &c. On the other hand, the reticulated tissue 

 of Falceacis is not in any way divisible into concentric and radial 

 elements, but is invariably irregularly and indefinitely trabecular, 

 and the entire substance of the skeleton appears to be more or 

 less conspicuously traversed by minute branching microscopic 

 tubuli. There would not appear, therefore, to be any very close 

 relationship between Palceacis and Stromatopora. 



5. To Hydrozoa. — AVith regard to the reference of the Stroma- 

 toporoids to the Hydrozoa, we have to consider on the one hand 

 their relationships to forms like 31illepora, and on the other hand 

 their affinities with H.ydractinia. The likeness of certain of the 

 Stromatoporoids to Millepora is very striking, but cannot be said 

 to be sustained by a close examination, the arguments on this 

 head being very much the same as those above referred to in con- 

 nexion with the " Madrep or aria perforata." Though generally 

 like one another, no Stromatoporoid can be shown to consist of 

 two definite systems of larger and smaller tubes, both divided by 

 tabulae ; and none can be shown to possess a definite system of 

 tubular cavities which can be supposed to have been tenanted by 

 zooids, while many are destitute of even a semblance of struc- 

 tures of this nature. The only Stromatoporoids which would 

 afi'ord a reasonable basis for a comparison with Millepora, or with 

 any of the so-called " Tabulate Corals," are those which form the 

 genus Caunopora. If the structure of these has been rightly 

 interpreted by us, and if the large vertical tubes of Caunopora 

 really belong to the organism of which they seem to form a part, 

 then we certainly have here a Stromatoporoid which may very 

 fairly be compared with Millepora, or, indeed, with any of the so- 

 caQed "Tabulate Corals." Caunopora, however, if its structure 

 be rightly interpreted, is an aberrant form, and it cannot by itself 

 decide the systematic position of the Stromatoporoids generally. 



In 1876, Dr. Grustav Lindstrom (Ann. & Mag. Nat. Hist. ser. 4, 

 vol. xviii. p. 4), in a paper on the " Anthozoa Tabulata," pointed 

 out that the well-known Silurian fossil referred by Milne-Edwards 



