TIIECOCYATHUS. 13 



THECOCYATHUS M.-Edw. & H. 



This genus, established by Mihie-Ed wards and Haime, contained 

 heretofore only three species, fossils of the Upper Lias ; there is 

 but one other genus of Turbinolidas as old as this one, the flimily 

 having received its greatest development in the tertiary and present 

 epochs. No representative of the genus Thecocyathus is known from 

 any of the formations intermediate between the Lias and our epoch. 

 Recent forms being found now present, therefore, a rare instance of the 

 reappearance of a genus apparently extinct through a considerable 

 succession of ages. 



The two species described below agree in their shape, which is that 

 of a short cylinder. To maintain this shape, notwithstanding the 

 increase of the diameter of the calicle, a peculiar arrangement is 

 necessary. From every one of the interseptal chambers starts a 

 hollow root, homologous to those of Rhizotrochus, but never detached, 

 nor visible externally. These roots form a concentric circle round 

 the original base of attachment. As the coral grows new roots are 

 formed higher up, and a new circle concentric with the preceding is 

 formed. In full-grown specimens five or six of these circles are 

 produced. (See Plate V., figs. 3 and 4.) All the interseptal cham- 

 bers emit these canals (which are filled up by extensions of the polyp 

 body), thus differing from Rhizotrochus, in which only twelve chambers 

 are thus provided ; in our JR. fragilis only once during the whole 

 growth, in R. ijums apparently twice. 



A similar but less pronounced arrangement exists in Paracyathus, 

 and probably in other corals. 



Li the Bulletin of the Museum of Comparative Zoology, No. 7, I 

 had confounded two species under the name of T, cylmdracens . I 

 have since recognized the differences, after examining a pretty large 

 number of specimens. 



Thecocyathus cylindraceus Pourt. 



Thecocyathus cylindraceus Pouut. Bull. Mus. Comp. Zool., No. 7. 



Plate 11., figs. 14 and 15. 



Corallum attached by a broad base, short, cjdindrical. Costoe gen- 

 erally visible through the epitheca, which reaches to, and sometimes 

 over, the border of the circular calicle. Fossa shallow. Septa very 



