6 



LINDSTROM, ON THECOCYATHUS NATHORSTI. 



(figs. 6 — 8). It is, however, most probable, considering the evident 

 presence of an epitheca outside the septa in other species of 

 Thecocyathus, tliat there is an exceedingly thin epitheca, wliich 

 has been so closely united with the theca as to be inseparable. 

 Both specimens are somewhat irregularly grown, especially 

 the larger one (B), and have at a certain period of growth coarc- 

 tated the circumference of the calicle and formed, as it were, a 

 new calicle with a new theca within the old calicle (fig. 6) anä 

 then again enlarging to the former size. The new theca is 

 formed across the septa of the old calicle which continue with 

 their inner edges in the new one. This change in growth is 

 the same, that is so common araongst the palgeozoic corals and, 

 as I suppose, quite distinct from the phenomena of gemmation. 

 Similar changes are also seen in specimens of Thecocyathus 

 mactra Goldf. from Lias. 



Fig. 1 — 5 from specimen A; all, except f. 1, magnified f. Fig. 1. side view of 

 specimen A, nat. size. P. 2. The calicle. I — I denotes the septa of the first 

 cycle, a — b the two primary septa, seen froin the sides in fig. 3; c. Septum of 

 the third cycle, seen from the side in fig. 4. F. 3. Vertical section of the calicle, 

 in the direction of a — b of fig. 2; showing the so called piili and columellar papillse. 

 — F. 4 vertical section of a tertiary septum, c in fig. 2. — F. 5. Horizontal 

 section of the coral near the basis. 



. The larger specimen (B) has 7 millims. in length and 6 

 millims. at its widest part. The smaller specimen (A) is 5 

 millims. in both dimensions and 2 millims. at its flat basis, 

 with which it has probably been fixed (fig. 1). 



There are no costae. The circular calicle is rather deep. 

 The septa are in six systems of four cycles; the fourth cycle is, 

 however, not developed in one of the systems. 



