On Solenopora compacta, BilUngs sp. 531 



of very irregular shape. The walls of the tubes are singularly sinuated, 

 or folded into minute undulations (PI. XIII. Fig. 2), reminding us of 

 what is seen in long sections of Monotrypa undulata, Nioh., and other 

 allied forms. One of the most striking features shown by thin 

 sections is the rapid increase of the tubes by fission. This shows 

 itself in tangential sections by the common occurrence of projecting 

 septa-like processes, which extend into the interior of the tubes 

 (PI. XIII. Figs. 3, 5-8). It was the appearance of these septiform 

 processes which originally induced us to refer the fossil to 

 Tetradium ; aud the number of them which may be present 

 varies greatly. Sometimes they can hardly be recognized as existing 

 at all. At other times (PI. XIII. Fig. 3) they occur in scattered tubes, 

 here and there among the ordinary tubes, as is so commonly seen 

 in the genus Ghcetetes, Fischer. Again, in many of the specimens 

 from Ayrshire and Esthonia, these structures are extraordinarily 

 numerous (PI. XIII. Figs. 7, 8), giving to cross-sections of the 

 tubes precisely the appearance, on a small scale, presented by cor- 

 responding sections of the tubes of Laceripora, Eichw. The fissipa- 

 rous development of the tubes of Solenopora compacta is quite as 

 easy to recognize in longitudinal sections (PI. XIII. Fig. 2), as it is 

 in tangential slices. 



Cross-partitions in the tubes, or " tabulae," are unquestionably 

 present; but they are very variable in number. We have seen no 

 examples in which they are so abundant, or so regularly placed, as they 

 are said by Sir William Dawson to be in some of his specimens. In 

 sections of some of our examples especially if they are reduced to great 

 tenuity, they are not to be recognized as occurring at all. In most 

 specimens there are a few of these structures present, irregularly 

 developed, and commonly situated at the point where a tube divides 

 into two (PL XIII. Fig. 2), or where a concentric line of growth is 

 produced (PI. XIII. Fig. 9). The walls of the tubes, as examined in 

 long sections, are certainly imperforate. Certain specimens, how- 

 ever, show a peculiar granular structure of the walls, which in some 

 respects is very similar to what is observed in the skeleton-fibre of 

 certain Stromatoporoids. In a great many examples, further, the tubes 

 as seen in long sections (PI. XIII. Figs. 2 and 4) exhibit a peculiar 

 appearance, as if the walls were formed of dark, ill-defined granules 

 arranged in transverse lines, these lines corresponding in contiguous 

 tubes, and being separated by corresponding clear lines, and thus 

 giving rise to a fine concentric striation. The appearance here 

 alluded to is not at all unlike that presented by thin sections of the 

 massive Nullipores, when the component cells may not be very 

 distinctly shown : but we shall speak more particularly of this point 

 immediately. 



The above being the general structure of the skeleton in Soleno- 

 pora, it does not seem unnatural that it should have been referred 

 to the Actinozoa. There is not, in fact, anything whatever in the 

 main features of the fossil which would obviously distinguish it 

 from a species of the genus CJioitetes, Fischer, or Tetradium, Dana. 

 Tangential sections of certain specimens of Solenopora compacta, Bill., 



