98 BRITISH STROM ATOPOROIDS. 



Actinostroma ; and it appears to have various structural relationships with the form 

 which I described as Syringostroma densum. Pending fuller investigation, I shall 

 therefore place it under the head of Syringostroma. The validity of this, however, 

 either as a genus or sub-genus, will, of course, depend upon further and more 

 exhaustive researches into the minute structure of the type-species, S. densum, Nich. 



Fam. 4. Idiostromim?, Nich. 



The ccenosteum in this family is dimorphic, and the general skeletal tissue is 

 in the main reticulated, but the radial pillars and concentric laminae are usually 

 developed as clearly distinct structures. The skeleton-fibre may be porous, or 

 tubulated, or apparently compact (AwvpMpora). Definite zooidal tubes, often 

 extensively tabulate, are generally present. There are also present larger tubes, 

 likewise tabulate, and sometimes furnished with distinct walls. These may be 

 distributed irregularly through the ccenosteum ; but they more usually form a 

 single or multiple " axial tube," which gives off lateral tabulate branches. The 

 typical form of the ccenosteum is that of a cylinder, sometimes simple, sometimes 

 branched, sometimes fasciculate, but in other cases the skeleton may be massive 

 or spheroidal. Astrorhizaa do not appear to be developed. 



I propose to provisionally group together under the above head the four genera, 

 Idiostroma, Winch., Hermatostroma, Nich., Stachyodes, Barg., and Amphipora, 

 Schulz. It must be admitted that this arrangement is in important points not a 

 natural one, and it is very probable that further researches may render its modifi- 

 cation necessary. If we were to take the typical cylindrical or dendroid examples 

 of Idiostroma, Stachyodes, and Amphipora, we should have a compact group of 

 forms, distinguished by the shape of the colony, and by the possession of a main 

 axial tube furnished with tabulaa, and connected with smaller tabulate offshoots. 

 Both Idiostroma and Stachyodes, however, occur in massive or sub-massive forms, 

 and in these there is no principal axial tabulate tube, but there are a number of 

 such tubes irregularly distributed through the colony. This, in fact, is the only 

 character of importance Avhich would separate the massive examples of the former of 

 these two genera from Stromatopora, Goldf. Then, again, the relationship between 

 the massive forms of Idiostroma and the type which I have named Hermatostroma, 

 is too close to allow of their separation to any distance from one another ; though 

 in the latter the characteristic large tabulate tubes of Idiostroma are either appa- 

 rently wanting or are present only in a modified form. The type which Bargatzky 

 named Stachyodes is in its general features very similar to the cylindrical forms of 



