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DR. A. VATJGHAN ON THE PALJIONTOLOGICAL [May I905, 



The secondary septa are rudimentary, and are only represented, 

 in a horizontal section, by a slight projection of the wall between 

 two primary septa ; they are, however, more conspicuous in the 

 calyx. 



Grouping of the primary septa. — On each side of the 

 fossula there is a lateral group, composed of from five to seven septa, 

 which unite at their inner ends to form the walls of the fossula ; 

 the remaining (thirteen to fifteen) septa form a nearly-continuous 

 antifossular series. There is a single, long but thin, fossular 

 septum. 



Septal breaks and fossula. — The fossula forms a conspicuous 

 break between the two lateral groups. Between each lateral group 

 and the antifossular group, there is usually a less-marked septal 

 break indicated by : — 



Fig. 2. — A tabula of Zaphrentis 



off. Phillipsi, 

 contour. 



represented by 



LcL 



Id 



(1) The stronger development of a 

 secondary septum, which bends 

 round and • unites with the 

 antifossular group. 



(2) The stronger development of the 

 two terminal septa of the anti- 

 fossular group, which frequently 

 join across the centre of the 

 section, so as to bisect it. 



This feature is extremely 

 marked in the very young 

 stage, when the fossula is in- 

 conspicuous. 



a.d. = Antifossular depression. 

 l.d. — Lateral depression. 



There is, usually, some indica- 

 tion of an antifossular septal 

 break. 



The tabulae have a complex 

 flexure, but their essential cha- 

 racter is that of a dome with 

 four grooves down its sides, as 

 represented in the accompanying 

 contoured diagram (fig. 2). 

 Discussion. — The original figure of Zaphrentis Phillipsi in the 

 'Polypiers Foss. Paleoz/ pi. v, fig. 1, differs from the Bristol form 

 in the presence of : — 



(1) More numerous septa, which are consequently less widely spaced. 



(2) Greater radial symmetry. 



(3) Indistinct lateral septal breaks. 



On the other hand, the description given in the ' Monogr. Brit. 

 Foss. Cor.' pt. iii, was largely founded upon specimens in the Bristol 

 Museum, which were derived from the Mendip area, and are quite 

 typical Bristol forms. The authors attribute the differences between 

 this form and their type to the suggestion that all the British 

 specimens that they had examined were young forms. From the 

 examination of thousands of specimens, I am, however, convinced 

 that the above differences are constant, and represent true varia- 

 tions of the adult stage. The position of the fossula is emphasized 



