121 



corallum as cost a-. These, in the species referred to, as well as in others, 

 commonly sliow tiie peculiar tetrameral arrangement characteristic of the 

 septa of this group. On or near the convex longitudinal surface of the 

 corallum a median, or "cardinal," septum appears, from which the 

 secondary septa pass off in a pinnate manner. (Fig. 2a.) Ninet_v degrees 

 towards either side occur the "alar" septa. (Fig. 2b.) These are par- 

 allel * to the secondary septa whicli branch off from the cardinal septum. 

 They have a single series of secondary septa branching off from them on 

 the side away from the cardinal qiiadrantx. The two remaining, or 

 counter quadrants, are filled with parallel septa, which branch off, in a 

 pinnate manner, from the alar septa, and are completed in front by the 

 counter septum, to which they are all parallel. 



One of the four " primary septa " — commonly the cardinal septum — 

 may be aborted, leaving a groove or fossula. Between the septa various 

 endothecal tissues may be developed, such as cross-plates, or dissepiments 

 connecting adjoining septa; tabula' or floors, more or less dividing off 

 the whole inner space, irrespective of the septa; and ci/sts, which form 

 a vesicular tissue more or less regularly disposed (Cystiph3ilura). The 

 cup or calyx may be limited below by a continuous floor, by dissepiments, 

 or otherv^ise, or it may be limited only by the margins of the septa, the 

 spaces between the septa being open to the bottom of the corallum. The 

 costaj are commonly covered by a concentrically wrinkled cpif/iera. which 

 forms the outermost wall of the corallum. 



In colonial forms the adjacent corallites commonlj'' become prismatic 

 from crowding. The separate thecae maj^ be retained, or they may 

 become obsolete, the corallites becoming confluent. The epithecal cover- 

 ing in these forms is commonly confined to the free margins of the outer 

 corallites, and surrounds the whole colony as a peritheca. 



Reproduction takes place by ova and by budding. In certain aberrant 

 forms, e. g. , Farositidn', MonticuUporida', etc., the septa are obsolete, 

 or nearly so, but tabuke are well developed.! 



Note. — An account of the structure of the polyji may be obtained from 

 any text-book of zoology. Zittel's Text-book of Palaeontology (East- 

 man's translation) should be studied for this as well as the succeeding 

 groups. The descriptions of species are still very imperfect, especially 

 as far as the Devonian species are concerned. The only available work 

 treating of them is the now rare volume in the Pakvontology of New 

 York series, entitled "Illustrations of Devonian Fossils," and even 

 this is incomplete, inasmuch as it is unaccompanied bj' descriptions. 

 The various sources from which the following descriptions are drawn 

 are indicated in the references under each species. The most complete 

 work extant is that of Milne Edwards et Ilaime ; Histoire Naturelle des 

 Corallaires, 3 volumes, and atlas; Paris, 1857-60. This classic the student 

 will do well to consult ; also the " Monographie des Polyj/iers Fossiles des 

 Terrains Pakmzoiques," by the same authors, Paris, 1851. 



* Parallel as seen iu the coster. 



+ As no true HexacoraUa occur in the formations treated of in these pages, an account 

 of their structure is omitted. 



