HALYSITID/E. 



339 



V^'/TT^ 



HALYSITIDiE. 



This family comprises only the familiar Ordovician and Silurian 

 corals known commonly as " Chain-corals," and constituting the 

 single genus Halysites. The corallum 

 in Halysites (fig. 219) is fasciculate and 

 reticulate, composed of long, tubular, 

 cylindrical corallites, which are placed 

 side by side in intersecting and anasto- 

 mosing laminae or lines, any given coral- 

 lite being united with its neighbours to 

 the right and left along its whole length, 

 and each lamina of the corallum con- 

 sisting of no more than a single linear 

 series of tubes. Each tube is enclosed 

 in a strong imperforate wall, surrounded 

 on its free sides by a thick epitheca ; 

 and there may be a distinct division of 

 the corallites into two series of different 

 sizes, in which case a single small tube 

 is placed between each pair of the larger 

 tubes. In other cases all the tubes are 

 similar in size and structure. Septa may 

 be wanting, and, when present, have the 

 form of vertically disposed rows of spines, 

 in cycles of twelve. The tabulae are 

 complete, horizontal, or slightly concave, not vesicular, the smaller 

 corallites (when present) being much more closely tabulate than the 

 larger ones. 



The species of Halysites may be divided into two groups, according 

 as the corallum is composed throughout of corallites of one size, or 

 consists of two sets of corallites of different sizes. The common 

 H. escharoides of the Silurian rocks (fig. 219, and fig. 220, a) is an 

 example of the forms in which all the corallites are similar. On 

 the other hand, in the familiar H. catenularia of the same formation, 

 the corallum consists of large corallites separated by the intervention 

 of small, closely tabulate tubes (fig. 220, b and c). Septa are some- 

 times absent (fig. 220, b), but in other instances they are well devel- 

 oped (fig. 220, a). When present, the septa always have the form 

 of vertical rows of spines, twelve of such rows being usually recog- 

 nisable in each corallite. The mode of growth of the corallum is 

 by " stolonal gemmation/' the colony in its early condition being not 

 unlike that of an Aidopora. 



The zoological position of the genus Halysites cannot be regarded 

 as certain, but it would appear, on the whole, to be most nearly 



Fig. 219. — «, Upper surface of a 

 fragment of Halysites escharoides, 

 from the Silurian rocks of Canada, 

 of the natural size ; b, Part of the 

 same enlarged. (Original.) 



