212 PEOF. H. A. NICHOLSON AND DE. J. MUEIE ON THE 



toporoids which have grown parasitically upon colonies of Syringo- 

 pora. At the same time, we desire to speak with diffidence upon 

 this point, as we should require more material thaa has as yet 

 been available to us before arriving at a final and positive conclu- 

 sion*. 



8. Concentric Arrangement of tlie Chamhers round Vertical 

 Columns. — Most of the Stromatoporoids show a tendency to a 

 concentric arrangement of the horizontal laminae round an indefi- 

 nite number of minor centres, this being especially well seen in 

 weathered and silicified specimens, though others exhibit little 

 or nothing of this peculiarity. In Syringostroma columnar is ^ 

 Nich., however, and in an undescribed Stromatoporoid (which we 

 shall name Stylodictyon retiforme) from the Hamilton formation 

 of Canada, this tendency is especially w^ell marked, and is accom- 

 panied by the feature that the minor centres round which the 

 chambers are arranged have the form of vertical columns, which 

 run through the mass in a perpendicular direction and attain a 

 very considerable size (from | to 1 line in diameter). In the 

 Hamilton species just mentioned, these columns (which must not 

 be confounded with the vertical pillars crossing the interlaminar 

 spaces) are composed of reticulate calcareous matter, more dense 

 by far than the general mass, and sometimes traversed by a ver- 

 tical canal in the centre. In Syringostroma columnaris the columns 

 are composed centrally of granular calcareous matter, apparently 

 quite imperforate and not reticulate, placed on an average about 

 one line apart. Whatever be the minute structure of the columns, 

 the chamberlets are arranged round them (as seen in horizontal 

 sections) in from two to four concentric rows, the interspaces be- 

 tween these being filled up with the ordinary reticulate structure 

 of Stro7natopora. 



9. Departures from the ordinary Type o/'Stroraatopora. — All the 

 typical forms of the Stromatoporoids, as we have seen, are com- 

 posed essentially of a system of more or less porous or reticulate 

 horizontal layers separated by interlaminar spaces, which in turn 

 are crossed by numerous solid calcareous pillars having a vertical 

 or radial direction. There are, however, several departures from 



* Since the above was written we have obtained a very extensive series of 

 CaunoporcB from the Devonian Limestone of Devonshire. These we have not 

 yet had time to investigate microscopically ; but an examination of their cha- 

 racters, especially as shown by cut and polished specimens, enables us to affirm 

 with certainty that the tubes of Caunopora unquestionably belong to the or- 

 ganism itself, and that they have no connexion whatever with Syrivgoporn. 



