NORTH AMERICAN INDEX FOSSILS. 



Fig. 75. Kunth's dia- 

 of tetrameral coral, 

 counter; 



varies in form and texture. The twisting of the septa at the center 

 may give the appearance of such a structure (pseudocolumella). 



In Palaeozoic corals the arrangement of the septa is often clearly- 

 seen to be in four groups (letracoralld) distributed in correspond- 

 ing quadrants. This arrangement is frequently well marked by 

 the course of the lines which on the exterior indicate the division 

 between the septa. A main or cardinal septum is distinguishable, 

 from which at either side branch off secondary septa in a pinnate 

 fashion (Fig. 75 , h). These occupy the two cardinal quadrants, and 

 are limited by the alar septa on the sides of the corallum ( Fig. 75 , s). 

 From the alar septa branch off other secon- 

 dary septa in a pinnate fashion on the side 

 away from the cardinal septum, these filling 

 the two counter quadrants which are divided 

 from each other by the counter septum, which 

 lies directly opposite the cardinal septum 

 (Fig. 75,£"). The cardinal septum is frequently 

 reduced so as to leave a marked groove or 

 fossula. The secondary septa are often united gram 



... . , , . . _ h, cardinal 



to each other in such a manner that their united ^ ^ lateral or alar septa# 

 ends form a continuous wall around the fos- 

 sula, or they may unite in groups, one on either side of the cardi- 

 nal septum and one near each alar septum. The apparent 

 grooves thus produced are the pseudofossulce. This wall may some- 

 times close completely so as to form an inner tube, into which the 

 septa do not extend. Tertiary septa often not reaching the center 

 occur. In most Mezozoic and later corals the septa occur in mul- 

 tiples of six, and are often very numerous, most of the cycles of 

 septa reaching the center. 



In compound forms the corallites may be crowded, when they 

 are generally prismatic in form, or they may be far apart, remain- 

 ing either entirely dissociated at the upper surfaces, or being con- 

 nected by a dense or coarse calcareous tissue — the ccenencliyma. 

 Sometimes the corallites are connected merely by their strongly 

 developed costae. In a few cases they become confluent by in- 

 complete division, and present the character of winding grooves 

 with the septa pointing to the center of the groove. When the 

 septa are absent tabulae are generally well developed (Favosites) 

 which in some forms may be* funnel-shaped (Syringopora). The 

 walls of such corallites are frequently pierced by a more or less 



