34-0 CHARACTERS AND DIVISIONS OF ALCYONARIA. 



related to the Heliolitida. At the same time, there are indications 

 of a possible relationship between Halysites and the Syringofioridce. 

 This is particularly shown by the peculiar coral described by Mr 

 Etheridge from the Arctic Silurian rocks under the name of Haly- 

 sites catenulatus, var. Feildeni, which has the form of Halysites, but 



Fig. 220. — A, Transverse section of Halysites escharoides, Lam., from the Silurian of Gotland, 

 enlarged ten times, showing the spiniform septa, and the absence of a series of small corallites ; 

 b, Transverse section of Halysites catenularia, Linn., from the Wenlock Limestone of Dudley, 

 enlarged five times, showing the absence of septa, and the presence of a series of small corallites ; 

 C, Vertical section of a few corallites of the last specimen, enlarged five times, showing the large 

 and small corallites, and the difference in the tabulation of these. (Original.) 



in which the corallites resemble those of Syringopora in being fur- 

 nished with connecting-tubes. This singular type cannot, however, 

 be regarded as properly belonging to Halysites. 



Tetradiid^e. 



This small family includes only the single genus Tetradium, 

 which, so far, has only been detected in the Ordovician rocks of 

 North America. In this genus the corallum (fig. 221) is massive, 

 and is composed of long, prismatic and closely contiguous corallites, 

 which are of small size, and have imperforate walls. The tubes are 

 furnished with longitudinal inflections or plications, generally four in 

 number in each tube, which do not reach the centre of the visceral 

 chamber, and which give a characteristic cruciform or petaloid aspect 

 to cross-sections of the corallites (fig. 221, b). These longitudinal 

 plications are apparently to be regarded as of the nature of septa 

 or pseudosepta. The tubes are crossed by numerous tabulae (fig. 

 221, c), which are sometimes complete, but are at other times per- 

 forated by central apertures (as in the tabulae of some species of 

 Stenopora). The mode of increase appears to be by fission. 



Tetradium appears to be related to Halysites, but its true affinities 

 and zoological position are uncertain. 



