356 



Close to the extreme point of the base the septa and dissepiments have an 

 irregular arrangement, and the loculi sometimes here appear to be mere 

 circular perforations. 



The small branching spicula, above figured, are seen imbedded in and 

 forming a part of the substance of the outer wall of A. Minganensis. The 

 fusiform and cylindrical varieties are also seen, but rarely, either adhering 

 to or partially imbedded in the same specimens. As they were obtained 

 in thousands in the sediment left after dissolving pieces of the limestone 

 holding fragments of this species, they were at first thought to belong to it. 

 But I have recently, while treating other pieces of limestone from the same 

 bed, also holding fragments of A. Minganensis, found that another large 

 species, Trichospongia sericea, occurs in this rock, portions of which are 

 crowded with, and seem to be almost altogether composed of, these spicula. 

 It should therefore remain an open question whether or not these fusiform 

 and cylindrical spicula actually form a part of the structure of Archeo- 

 cyatlius, or are those of T. scricea. There can scarcely be any doubt 

 about the branched spicula, as they can be seen not only projecting from 

 the surface of the silicified specimens, but also in the thin slices prepared 

 for the microscope. No spicula have been detected in A. profundus. 

 In A. Atlanticus there are several objects visible, in the only specimen of 

 that species that has been collected, which resemble branched spicula. No 

 silicified specimens of these two latter species have been procured, and I 

 think it probable that if such could be examined spicula would be found in 

 them. 



As to the zoological rank of this genus there yet remains some doubt. 

 The general structure is such that it may possibly be a sponge. The 

 apertures in the external wall may be the homologues of the inhalent pores 

 of the ordinary sponge, while those of the inner wall may represent the 

 exhalent orifices. The great internal cavity, in that case, would have the 

 same function as the large central cloaca of the fistulose genera of sponges. 

 A radiated and more or less perfectly septate structure occurs in many 

 undoubted sponges. In this genus, however, the substance of the septa is 

 almost as compact as that of the true corals. The perforated character of 

 the outer wall, and also of the septa, suggest a comparison with corals 

 of the division Zoantharia perforata. The Favosites have also their walls 

 perforated. Dr. J. W. Dawson, who has examined a number of the slices 

 of A. Atlanticus and A. profundus, which have been prepared for the 

 microscope, is of opinion that the structure of these two species is similar 

 to that of the Foraminifera.* My own opinion is that all three species 



* Can. Nat. and Geol. April, 1865. 



