290 



Family Reteporidae Smitt, char, emend. 

 (PI. X, figs. 1-5; PL XXIII, figs. 1-5). 



The zooecia, which are composed of a very hard and thick calcareous mass, 

 are as a rule only sparingly provided with pores and may have 2 — 8 spines, 

 which are not rarely composed of a row of internodes. There is a more or less 

 well-developed vestibular arch, which is usually beaded, and the separating walls 

 between the single zooecia are provided with uniporous, more rarely with few-pored 

 rosette-plates, which may sometimes be placed in the inner wall of small pore- 

 chambers. Each distal wall is usually provided with one and the distal half of 

 each lateral wall with one uniporous rosette-plate. Dependent avicularia of varying 

 form and size occur as a rule. The hyperstomial ocecia, which spring from a 

 narrow, sometimes almost stalk-like proximal part and consist in their whole ex- 

 tent of two calcareous layers, are originally free, though as a rule more or less 

 deeply immersed in niche-like depressions; but in older parts of the colony they 

 may often be quite hidden under the covering layer which grows over them 

 from the margins of the niche-like depression. Their frontal surface may some- 

 times be complete, sometimes provided with a slit-like or trilobed uncalcified 

 part, almost never with scattered pores. An ooecial operculum seems to be always 

 absent, but on the other hand the free margin of the ooecium is not rarely pro- 

 vided with a larger or smaller, obliquely inwards directed, median projection, which 

 serves to reduce the size of the aperture. The colonies are not rarely incrusting, but 

 usually free and in most cases occur in the form of a perforated network of sinuous 

 or folded laminae, which only consist as a rule of a single layer of true zooecia 

 (almost always directed towards the inner side of the colony). The opposite surface 

 of the colony is however covered by one or several layers of kenozooecia, the inner 

 cavities of which are greatly reduced or quite absent and which agree with the 

 zooecia neither in form nor in size. A larger or smaller number of these keno- 

 zooecia are provided with avicularia and the colony is fastened according to its 

 age and size by means of a larger or smaller expansion formed by similar keno- 

 zooecia. 



This diagnosis of the family is based on the investigation of a large number 

 of species, and the family is undoubtedly one of the most natural and most 

 sharply defined. Whilst the aperture, operculum and the peristome are subject 

 to large differences, we find as a rule distinct hinge-teeth and a more or less 

 well-developed vestibular arch, which is usually provided in the neighbourhood 

 of the margin with small, rounded projections. Such a crenulated arch can be 



