554 MR. E. E. L. DIXON AND DR. A. VAUGHAN ON [Nov. I9I I, 



Major septa very strongly thickened and frequently coalescent ; 

 minor septa only developed in the calyx. 



Horizontal sections.— In the young stage the horizontal 

 section of the fossula is regularly and broadly oval, and bisected by 

 a strong fossular septum (PL XL, fig. 1 b). 



In the adult, where the 



Fig. 9. — Zaphrentis oystermonthensis, septa have ceased to in- 



sp. nov. : section below the calyx, crease, the fossula is broadly 



magnified 2 diameters. (Specimen open near the wall and 



figured in PI. XL, Jig. la.) narrows to the centre, being 



bounded by a single septum 

 on either side (PI. XL, 

 fig. 1 e). 



In intermediate stages the 

 appearance of the fossula 

 depends on whether the sec- 

 tion intersects a pair of new 

 septa or not ; if it does so, 

 the fossula is contracted in 

 the middle, as in Z. dela- 

 nouei ; if it does not, the 

 appearance is much like that of the adult (compare PL XL, 

 figs. 1 c & Id). 



The septal plan has the following characters (see fig. 9, above) : — 



(1) A strong thickening of the counter-septum ; 



(2) Great variation in length and direction of the major septa ; 



(3) A strong convexity of the longer septa (in the adult) to the generative 



planes (or ' fossular breaks.') 



Erom (2) and (3) results the characteristic palmate grouping of 

 the septa and the dependent lateral boundaries of the counter 

 series. 



Tabulae (see fig. 10, p. 555). — The tabulae are broad, strongly 

 depressed in the middle and along the fossula, and gently everted at 

 the wall. 



Horizon and localities. — This species is abundant in D 2 _ 3 of 

 Oystermouth Quarry. A very similar form is rare in the ' Cya- 

 thaxonia Beds ' of Rush. 



Resemblances and differences. — Zaphrentis delanouei Ed. 

 & H. is of much smaller average dimensions, and its dominant form 

 is almost purely conical (only the tip being curved), whereas the 

 usual form of Z. oystermonthensis is continuously cornute. 



In Z. delanouei the counter (anti-fossular) septa in the adult are 

 regularly radial, uniformly thick, and approximately of equal 

 length; whereas in Z. oystermonthensis they present a quadri- 

 pinnate grouping, and are of very unequal thickness. 



Comparison with Z. enniskilleni Ed. & H. — The type- 

 specimen of Z. enniskilleni Ed. & H., represented in PL XL, fig. 2, 

 shows how markedly Z. oyster mouthensis differs in the greater 

 irregularity and thickening of its septa, although agreeing in the 

 general plan of septation and in the position of the fossula. 



