STROMATOPOROIDEA. 229 



tubes often show the appearance of short inward projections, which 

 have the aspect of the radiating " septa " of a coral, but which are 

 doubtless of a wholly different nature. 



The general structure of the skeleton in Stoliczkaria, and appar- 

 ently in Syringosphcera also, is thus in many respects similar to that 

 of the recent genera Allopora (fig. in) and Sporadopora among the 

 Stylasterids. These singular organisms may therefore be regarded 

 as being referable to the Hydrocorallines, and as being related to the 

 Sty laster idee. From these, however, they differ in their free habit, 

 and in the peculiar radial tubulation of the ccenosarcal tissue ; and 

 they may thus be properly considered as constituting a special family, 

 to which the name of Syringospkceridce, proposed by Professor 

 Duncan, may be given. The resemblance between Stoliczkaria and 

 Syringosphcera on the one hand and Parkeria on the other hand, 

 though superficially close, is not dependent on real identity of struc- 

 ture. They agree in the general nature of the ccenosarcal tissue ; but 

 the skeleton in Parkeria is composed of concentric layers, separated 

 by tiers of chamberlets or by " interlaminar spaces," and is traversed 

 by numerous radial pillars, while the zooidal tubes are irregular and 

 are devoid of a columellar style. 



Sub-class Stromatoporoidea. 



The Stromatoporoids constitute a large group of extinct Hydrozoa, 

 which, so far as certainly known, are confined to the Ordovician, 

 Silurian, and Devonian rocks. The ccenosteum in the forms in- 

 cluded in this group is calcareous and often of large size ; and 

 some of the limestones of the Silurian and Devonian period are 

 very largely made up of the skeletons of these organisms. The form 

 of the skeleton and the mode of growth are extremely variable, but 

 the most typical condition of the ccenosteum is that of a spheroidal 

 or irregular mass, or of a flattened expansion, attached basally to 

 some foreign body, and exhibiting a more or less conspicuous com- 

 position out of concentric calcareous laminae (fig. 1 1 4). In some 

 cases the skeleton is branched and dendroid, and in others it forms a 

 thin crust growing upon foreign bodies. In this latter case, as well 

 as in some massive types, the organism is attached by the whole of 

 its under surface ; but more usually the attachment is by a peduncle, 

 and the greater portion of the basal surface is covered by a thin, 

 concentrically wrinkled, calcareous membrane or " epitheca." 



As regards the general structure of the skeleton of the Stromato- 

 poroids, the most obvious feature is that it is composed of numer- 

 ous concentric layers or " laminae," which are separated by narrow 

 intervals or " interlaminar spaces." In reality, however, the actu- 

 ally fundamental element in the skeleton is not found in these con- 



