290 



Family Reteporidae Smitl, char, t-mend. 

 (PI. X, lij<s. 1-5; I'l. XXIII, figs. 1-.')). 



The zowcia, which are foni])oso(l of a very hard and tliick calcareous nias.s, 

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

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

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

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

 rosette-])lales, 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 lalcral wall with one unijiorous rosette-plale. Dependcnl (ivhiiUirin of varying 

 form and size occur as a rule. The hyperslomial ixvcin, which spring from a 

 narrow, sometimes almost stalk-like proximal ])art 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 |)ar(s of the colony they 

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

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

 times be complete, sometimes provided witli a slit-like or trilobed uncalcilied 

 j)art, almost never with scattered pores. An od'cial o])erculum seems to be always 

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

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

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

 usually free and in most cases occur in the form of a |)erforated network of sinuous 

 or folded laminte, which only consist as a rule of a single layer of true zoo'cia 

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

 of the colony is however covered by one or several layers of kenozoorid, 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 avicidaria and the colony is fastened according to its 

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

 zofccia. 



This diagnosis of the family is based on the investigation of a large nund)er 

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

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

 to large dilTerences, we (ind as a rule distinct hinge-lecth and a more oi' less 

 well-developed vestibular arch, which is usually |)rovide(l in the neighbourhood 

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



