358 Mr. E. G. Carruthers on the 



primary hexamerous plan in the Rugosa, but, on the contrary, 

 support that view. 



There seems to Le no doubt that tlie unsatisfactory state of 

 our knowledge of these primary stages is due to lack of 

 sufficiently good material. There is, however, in the calca- 

 reous shales found in the Carboniferous Limestone Series of 

 Scotland an abundance of small corals, chiefly Zaphrentids, 

 and often beautifully preserved. Large numbers of these are 

 in the collections of the Geological Survey, and in dealing 

 with them during a revision of the corals for some forth- 

 coming Sheet Explanations, so many were found to show the 

 earliest stages of their septal development that opportunity 

 was taken to investigate the matter further. The majority 

 were so preserved that on carefully grinding down the tips, 

 the septal arrangement could easily be seen with the aid of 

 a hand-lens. This fact to some extent obviated recourse to 

 microscopic sections, but of the latter about one hundred in 

 all were made in order to permit of more detailed examination 

 of the various stages. From the Museum of Practical 

 Geology in London one or two additional Carboniferous corals 

 and also several Silurian ones were obtained ; while, owing 

 to the kindness of Mrs. Gray, 1 have been able to examine 

 from her unique collection of the Lower Palaeozoic fauna of 

 Girvan some small specimens of Streptelasma showing these 

 early stages. They are from the Silurian (Llandovery) rocks 

 of Woodhall Point and from the Ordovician (Llandeilo) beds 

 of Craighead. 



Sequence of Septa. 



In all the corals examined the septa were found to appear 

 in the same order and manner in both early and late PaliX?o- 

 zoic forms. As an example, one of the small Carboniferous 

 Zaphrentids will be chosen for description. The gi'owth of 

 the first six septa may conveniently be divided into three 

 stages, but it is to be understood that these stages merge 

 gradually into one another ; it is not till all six septa have 

 appeared that any pause in the development is met with. 

 On carefully grinding down the tip of the coral the septal 

 sequence is seen to be as depicted in Diagram A. 



Stage L — A single septum is seen to stretch across the 

 calicle from wall to wall (diag. A. 1). This may conveniently 

 be referred to as the "axial septum.-" In later stages this 

 "axial septum^' breaks up to tbrm the main and counter 

 septa of the mature coral. Microscopic sections show that 

 the median dark line seen in most septa occurs here also in a 

 continuous dark band passing down the centre (lig. 1, PI. IX.). 



