28 



J. E. DUERDEN. 



THE ALAR OR LATERAL FOSSUL/E. 



We may first enquire into the nature of the alar or lateral fos- 

 sulae, as represented in the calice of a rugose coral such as Ha- 



o- 



FIG. I. Calice of Hadrophyllitm panciradiatnm, E. & H., showing the arrange 

 inent of the septa. Here and throughout the figures the Roman numerals I indicate 

 the six primary septa or protosepta, the letters a-e the secondary septa or metasepta, 

 and x the exosepta making up the outer smaller cycle (see description, Fig. 7) ; the 

 lower side is regarded as ventral and the upper as dorsal. The two alar fossulse 

 are formed by the medio-lateral group of shorter septa, c-e, on each side, and the 

 cardinal fossula by the ventro-lateral group of shorter septa, b-e, on each side, along 

 with the more depressed ventral directive septum (I). 



The series, Figs. 2-1 1, shows the complete septal development of Streptelaswa 

 rectum, Hall. The drawings were made as the successive stages were exposed on 

 grinding down a corallite ; the middle dotted line in each septum represents the line 

 or centers of calcification. 



drophyllum paudradiatum (Fig. i). The Devonian genus Ha- 

 drophyllum here chosen is one which Milne-Edwards and Haime, 



