234 APPENDIX. 



the large tubes, and by the presence of " spiniform corallites." I need 

 only add that the incomplete condition of the walls of the interstitial 

 corallites, and the often vesicular nature of their tabulae, are no grounds 

 for supposing that we have to deal here with a genuine " coenenchy- 

 ma ;" since precisely similar conditions are found in species oi Plasvio- 

 pora and Propora, where there is no room for reasonable doubt that the 

 small tubes are a peculiarly modified series of corallites. 



Dittopora, Dybowski. 



The name of Dittopora has been proposed by Dybowski for a group 

 of Monticuliporoids to which he assigns the following characters : 

 Corallum polymorphic ; corallites of two kinds, one series being cylin- 

 drical and separated by a reticulate coenenchyma, and the other con- 

 sisting of irregularly prismatic tubes which are in close contact. Both 

 kinds of corallites are either arranged in alternating zones, or are con- 

 fined to particular parts of the colony. " Spiniform corallites " (" Wand- 

 rohrchen ") are developed in connection with both kinds of corallites. 

 Tabular are present. 



The typical species of Dittopora, Dyb., are distinguished by a pecu- 

 liar form of dimorphism, certain portions of the corallum being occupied 

 by corallites which difier in form from the rest. Thus in D. clavccfor- 

 mis, Dyb., the lower pedunculate end of the clavate corallum is occu- 

 pied by prismatic corallites, while the upper bulbous portion is occu- 

 pied by cylindrical corallites which are separated by 

 ,cri~! j::::! sub-angular interstitial tubes. On the other hand, in 



p;-, ^ «^ • .•'^ D. annulata, Eichw. sp., the different kinds of coral- 

 1 ' ' • lites are arranged in zones, thick-walled cylindrical 



A ■ ^ tubes alternating in successive rings with thin pris- 



y«, « *' "^1 matic tubes (fig. 49). 



''^^^B*.,^,.,.-- As regards the minute structure of the corallum, 



it would appear that the bulk of the corallum is 



Fig. 49. — A fragment , ^ . , ... ^ , . 



of the corallum of "''^"^^ "^P O' Circular corallites of large size, separated 

 Dittopora anmdata, by a series of angular interstitial tubes, which are so 

 Eichw. sp., from the largely developed as to entirely isolate the large 



Silurian of Russia, , , /r n r^-, , . , t i 



enlarged. (Copied ^"°^^ ^^^- 5^, A). 1 here also cxists an exceedingly 

 from Eichvvald.) well-developed series of circular "spiniform coral- 



lites," which are remarkable for their exceptionally 

 large size. In long sections (fig. 50, B) it is seen that the large cor- 

 allites are furnished with a very small number of remote horizontal 

 and complete tabulae ; whereas the small interstitial angular corallites 

 (the " coenenchyma " of Dybowski) are provided with numerous closely- 

 set horizontal tabulae, their walls being partially imperfect. 



