90 SIDERASTREA RADIANS. 



distinctly angulated at its peripheral end, and where two additional elements 

 were introduced in each chamber the periphery of the septum was strongly 

 bifurcated. In some few cases no separate fragments whatever would arise 

 within an interspace. The septum would then retain its original bar-like 

 form, but become longer and usually more rugged in outline, showing that 

 skeletal matter was being added. The deposition of new matter was usually 

 more forward in the axial chambers than in the lateral, but scarcely any 

 two polyps were alike in the detailed appearance assumed by the skeleton 

 at this period. 



Figs. 12, 19, and 20, on plates 2 and 4, represent the actual polyp or coral- 

 lurn during this stage, while fig. 9, d, p. 88, is an attempt to represent diagram- 

 matically the septal conditions at its close. It is a well-defined phase, and 

 important as showing the different methods by which the septa may increase 

 in length and complexity. Although, as above remarked, no two polyps were 

 exactly the same, a general plan was determiuable throughout the many 

 examples studied. The septa in all but the pair of ventral exosepta con- 

 sist of a simple, more central portion and the peripheral additions. The 

 former represents the enlarged primary septa seen in fig. 9, , p. 88, while 

 the latter are newer formations. In general the peripheral deposits consist 

 of two or more separate fragments, placed at an angle with the central bar, 

 the angle being greater in the exosepta than in the entosepta ; the entoccelic 

 additions are, in fact, nearly parallel with the primary entoseptuni, and thus 

 more strictly radial. The two axial or directive septa .are usually the most 

 strongly developed of the entire series, and remained thus to the end ; in 

 addition, the dorsal and the ventral frequently differ much in form from 

 one another. 



In all the figures it will be seen that the two ventro-lateral exosepta 

 remain simple, enlarging but little beyond their primary condition, and 

 the middle exosepta are likewise somewhat less developed than the dorso- 

 lateral. Thus a decided dorso-ventrality is indicated in the growth of the 

 peripheral elements of the skeleton, as was the case with the simple exosepta. 



For many weeks afterwards the only alteration in the septa consisted 

 in increased growth along the plan thus laid down. As the peripheral frag- 

 ments enlarged, the members of any group became fused with one another. 

 After the second month the development of the skeleton within the living 

 polyp could with difficulty be followed, owing to the complexity of the 

 internal tissues. Therefore, from this stage onward the septa will be studied 

 mainly from macerated corallites. 



Fig. 23, plate 4, is from a photograph of the skeleton of a polyp ten 



