THE MORPHOLOGY OF THE MADREPORARIA, VI 

 THE FOSSULA IN RUGOSE CORALS. 1 



J. E. DUERDEN. 



The term fossula-, as employed in the literature of rugose corals, 

 refers to a depression or pit in the calice, due to the smaller size 

 of the septa at that particular region. Generally only one fossula 

 is present in a corallite (Figs. 10 and 1 1), but there may be three 

 (Figs, i and 12), or rarely two or four. The presence of the one 

 or more pits gives a decided bilateral character to the calice, 

 which otherwise might be perfectly radial. 



The occurrence of one or more fossulae has always been re- 

 garded as an important characteristic of the extinct Rugosa or 

 Tetracoralla, nothing suggestive of such being found among 

 modern hexameral Madreporaria, and, as would be expected, 

 various explanations have been put forward to account for their 

 presence. The present contribution is an attempt to understand 

 the nature of the fossulae from the stages passed through in the 

 development of the individual corallite. 



Where only a single fossula is present it is situated towards 

 the ventral end of the principal axis of the calice, and where three 

 occur the two additional are lateral and symmetrically disposed ; 

 when present the fourth fossula is towards the dorsal extremity 

 of the principal axis. The single ventral fossula is the most per- 

 sistent and characteristic of the series, and may be known as the 

 main or cardinal fossula, or better, as the ventral -directive fos- 

 sula, since it is associated with the cardinal or ventral septum. 

 The two lateral are the alar fossulae, and are dorsal to the alar 

 septa ; the fourth is the counter or dorsal-directive fossula. 



'The first two parts of this series of papers appeared in the Johns Hopkins Uni- 

 versity Circulars, Vol. XXI., Nos. 155 and 157, and were reprinted in the Annals 

 and Magazine of Natural History, Ser. 7, Vol. X., May and August, 1902. The 

 third and fourth parts appeared in the Annals and Magazine of Natural History, 

 Vol. X., November, 1902, and Vol. XI., February, 1903; the fifth part in the 

 BIOLOGICAL BULLETIN, Vol. VII., July, 1904. The work is being carried out with 

 the assistance of an appropriation from the Carnegie Institution. 



Contributions from the Zoological Laboratory of the University of Michigan, 

 No. 96. 



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