620 



M0LLUSC0IDEA. 



The singular genus Alveolaria (fig. 463) may likewise be associated 

 with the Lichenoftoridtz — the only described species being the A. semi- 

 ovata of the Red Crag (Pliocene) of Britain. In this remarkable form the 

 polyzoary is massive and globose, consisting of numerous mushroom- 

 shaped discs or cups, which are arranged in radiating and diverging 

 lines, the cups of each vertical series springing directly from one another 

 (fig. 463, c) ; and the different groups being so disposed that they form 

 a series of thick concentric layers. Each concentric stratum is thus 

 made up of a number of discs which are firmly united by their edges, 

 but are free towards their bases ; and each disc is enclosed in a strong 

 calcareous membrane, the upper surface alone being free, and ex- 



Fig. 463. — Alveolaria semiovata, from the Red Crag (Pliocene) of Britain. A, A specimen of 

 the natural size ; B, A portion of the surface enlarged ; c, Part of a vertical section enlarged ; D, 

 Part of a tangential section enlarged. (Original.) 



hibiting the apertures of the cells. The laterally-united discs are poly- 

 gonal, and the surface (fig. 463, a and b) thus shows a number of poly- 

 gonal cups bounded by strongly elevated ridges, each of which, in turn, 

 exhibits a raised median line corresponding with the free edge of the 

 epithecal membrane. For the same reason, tangential sections (fig. 

 463, d) show that the tubes are divided into polygonal groups, each of 

 which is bounded by a strong external wall. The tubes themselves are 

 in close contact throughout, their mouths not projecting above the 

 general surface. The walls of the tubes are apparently completely im- 

 perforate ; but " closing-plates " or " tabulae " are often developed. 



The family of the Frondiporidce (Theonoida) includes forms in 

 which the polyzoary is composed of long contiguous tubes, the 



