54 



M.K. HOWARTH 



Fig. 2 Pachysphinctes bathyplocus (Waagenj, 16.5 m above base of lower marly part of Kilya Member (fauna 7), Naifa Cliff; CA995, xO.5. 



ending as very large wedge-shaped radial ridges at the largest sizes, 

 so that rib densities as low as 1 8 primary ribs per whorl occur at 400 

 mm diameter. When the primary ribs start to increase in size and 

 strength, three to five secondary ribs are associated with each 

 primary, either by multiple splitting or intercalation. As size increases 

 the association between primaries and secondaries becomes less 

 clear, then the secondaries gradually disappear so that the venter 

 becomes broad and smooth from 225-250 mm diameter. One to 

 three oblique constrictions on inner whorls tend to disappear beyond 

 150 mm diameter. 



Measurements. In ammonites it is traditional to measure whorl 

 breadth in the inter-costal space between ribs, rather than across the 

 tops of the ribs themselves. In a species like P. major that has very 

 heavy ribs at large sizes, such a measurement of whorl breadth 

 makes the whorls appear much less depressed than when the meas- 

 urement is taken across the tops of the ribs. In the measurements 

 below an extra figure is given for the whorl breadth across the tops 

 of the ribs (WbR) which better illustrates the real amount of whorl 

 depression. 



D 



Wh 



Wb 



WbR 



U 



Lectotype 210.0 



57.0(0.27) 



— 



74.0 (0.35) 



103.0(0.49) 



(from Waagen) 











Lectotype 210.0 



59.0(0.28) 



— 



66.0(0.31) 



104.0(0.50) 



(from pi. 54) 











CA1170 420.0 97.0(0.23) 100.0(0.24) 129.0(0.31) 242.0(0.50) 



CA1170 310.0 77.5(0.25) 93.0(0.30) 105.0(0.34) — 



CA1171 277.0 75.0(0.27) 75.0(0.27) 87.0(0.31) 143.0(0.52) 



CA998 310.0 74.0(0.24) 89.0(0.29) 99.0(0.32) — 



CA1005 235.0 66.0(0.28) 63.0(0.27) 70.0(0.30) 120.0(0.51) 



CA1157 187.0 49.5(0.26) 63.0(0.34) 68.0(0.36) 100.0(0.53) 



CA1014 108.0 31.5(0.29) 35.0(0.32) — 54.0(0.50) 



Remarks. The largest specimen figured hitherto is Waagen's 

 (1875: pi. 54) lectotype, which is still septate at its aperture at 210 

 mm diameter {test Spath, 1931: 489), at which size the massive 

 whorls and heavy ribs are not fully developed. The typical characters 

 of the species are well seen in the very large 420 mm diameter 

 Yemeni macroconch (CAl 170) figured in PI. 8, fig. 2, which seems 

 to be the largest example found so far. It is, of course, much smaller 

 than the largest known Pachysphinctes, which is the 1095 mm 

 diameter P. muelleri from Tanzania, briefly described above (p. 50). 

 At 420 mm diameter the ribs of CA 1 1 70 are very large, and are of 

 such high relief that the ratio of whorl height/whorl breadth meas- 

 ured across the tops of the ribs (97 mm: 129 mm = 0.75) is much 

 lower than the same ratio (97 mm: 100 mm = 0.97) when the whorl 

 breadth is measured in the inter-costal space. Slightly greater whorl 

 depression (ratio 0.74) is shown at 310 mm diameter in the same 

 specimen where the whori height and breadth are 77.5 and 105 mm 

 respectively. Even these large whorls are not as depressed as the 



PLATE 6 



Figs 1, 2 Pachysphinctes major S,^d\h. la, lb, upper marly part of Kilya Member (fauna 8), Wadi Kilya, CAl 157, xO.66. 2a, 2b, base of middle limestone 

 part of Kilya Member (fauna 7), Naifa Cliff, CA1014. 



