MADREPORARIA PERFORATA. 323 



the briefest description in this place. The corallum in Thecia is 

 generally in the form of a laminar expansion, furnished inferiorly 

 with a striated epithecal membrane. In their early condition, the 

 corallites are often oblique to the basal plate, and in this stage they 

 have thin and apparently distinct walls, the visceral chambers being 

 crossed by complete tabulae, and the general appearance being not 

 unlike to that of some species of Favosites. Very soon, however, 

 the corallites become perpendicular to the basal plate, and they then 

 appear to lose their distinct walls and to become united by the 

 intervention of a thick, vertically tubulated calcareous tissue, which 

 may be regarded as probably of the nature of a spurious ccen- 

 enchyma, and from which the walls of the individual corallites are 

 wholly inseparable. The surface, therefore, exhibits the stellate 

 calices of the corallites separated by an apparently dense interstitial 

 tissue, marked superficially by minute tubercles, and by radiating, 

 often vermicular, grooves which extend from one calice to another. 

 The visceral chambers of adjacent corallites are placed in direct 

 communication by means of well-marked, often bent, horizontal 

 tubes, which represent elongated mural pores. The septa are gener- 

 ally twelve in number in each corallite, and have the form of irregular 

 vertical ridges, with very broad bases, which only extend a short way 

 inwards into the visceral chamber, and appear sometimes to terminate 

 along their inner edges in blunt spines. The tabulae are complete, 

 horizontal, or slightly curved, and tolerably numerous. 



The known species of Thecia are Silurian, the most familiar being 

 the common T. Swindernana of the Wenlock Limestone. The 

 coral described by Rominger from the corresponding horizon in 

 America under the name of Thecia major appears to differ in im- 

 portant respects from the European species of Thecia, and would 

 appear to be nearly related to the Favositoid genus Laceripora. 



The family of the Thecidce, as above defined, is separated from 

 the Favositidce by the complete fusion of the corallites in the ter- 

 minal portion of their course, and by the peculiar nature of the 

 tubulated ccenenchymal tissue which separates adjoining visceral 

 chambers. The fact that the corallites have as a rule twelve septa 

 each, would support the view that Thecia is to be regarded as refer- 

 able to the Zoantharia rather than to the Alcyonaria ; and the pres- 

 ence of tubes directly connecting the cavities of adjoining polypes 

 would show that the genus belongs to the Madrefioraria Perforata. 



to the structure and affinities of Thecia. The exothecal tubules were previously 

 regarded as corresponding with the " mesopores " of Heliolites, and the genus 

 was on this ground placed among the Alcyonaria. The examination of well-pre- 

 served specimens by means of thin sections would, however, appear to prove 

 conclusively that this is not their nature, and that they are really what Milne- 

 Edwards and Haime described as a " spurious ccenenchyma." 



