56 



M.K. HOWARTH 



fragment that Waagen (1875: 193) said had a whorl height and 

 breadth of 65 mm and 100 mm respectively (ratio 0.65), from which 

 it has to be concluded that Waagen measured the whorl breadth 

 across the tops of the ribs. The example in PI. 6, fig. 1, though 

 roughly preserved, is figured to show that coarse ribs and depressed 

 whorls can develop much earlier, here from about 1 25 mm diameter, 

 and thereafter such specimens differ greatly from P. bathyplocus. 

 Coming from the upper, marly part of the Kilya Member in Wadi 

 Kilya, of Hybonotum Zone age, it is from the youngest horizon of 

 ammonites in the Naifa Formafion. Inner whorls of P. major are 

 figured in PI. 6, fig. 2, to show the close resemblance at these sizes 

 to P. bathyplocus, and in this example the large whorl thickness is at 

 least partly due to the distortion caused by preservation in a septarian 

 nodule. There are no clear microconchs amongst the Yemeni collec- 

 tion, but a fine Cutch microconch with lappets at 175 mm diameter 

 was figured by Spath ( 193 1 : pi. 97, fig. 1 : pi. 98, fig. 5). Collignon 

 (1959: pi. 1 18, fig. 446) had another microconch complete at 161 

 mm diameter amongst the three specimens (Collignon, 1959: figs 

 446, 449, 451) that he figured under three different names, even 

 though all came from the same bed and locality in the Hybonotum 

 Zone in Madagascar. It is the same bed from which came the four 

 examples of P. bathyplocum referred to above, and shows that as in 

 southern Yemen, the two species occur together at the same horizon. 



Occurrence. Lower, middle and upper parts of the Kilya Mem- 

 ber, Naifa Formation, Wadi Kilya, Naifa Cliff and Wadi Arus; 

 Beckeri Zone, Upper Kimmeridgian and Hybonotum Zone, Lower 

 Tithonian. 



Pachysphinctes mahokondobeyrichi (Dietrich, 1925) 



PI. 5, fig. 2; PI. 8 fig. 5 



1925 Perisphinctes (Virgatosphinctes) mahokondobeyrichi 

 Dietrich: 13, pi. 3, fig. 5. 



Lectotype. The original of Dietrich, 1925, pi. 3, fig. 5, here 

 designated. 



Material. Five specimens, CA1017 and SM F. 12190-93, from 

 the base of the middle limestone part of the Kilya Member in Naifa 

 Cliff. 



Description. All five specimens are fragments of body-cham- 

 bers, the largest having a whorl height of 37 mm, indicating a 

 diameter of approximately 140 mm. The whorls are quadrate and 

 slightly depressed. Dense primary ribs mostly bifurcate at the 

 ventro-lateral edge, and occasional ones trifurcate. No fragment is 

 long enough to count the number of ribs per whorl or per half whorl, 

 but ribs are more dense than in similar-sized examples of P. 

 bathyplocus, suggesting a rib-density in the range 55-60 per whorl 

 at 100 mm diameter. 



Remarks. P. mahokondobeyrichi is the identification given to 

 examples of Pachysphinctes that have whorl proportions similar toP. 

 bathyplocus, but are more densely ribbed. Dietrich (1925: 13, pi. 3, 

 fig. 5) quoted rib densities of 45 per whorl at 50 mm diameter and 63 

 per whorl at 95 mm diameter (though counting on his figure (which 

 is a retouched photograph ) gives densities of 5 1 at 46 mm and 64 or 

 65 at 93 mm diameter), and the Yemeni specimen appears to fall 



within this range. In other respects they do not differ from P. 

 bathyplocus. 



Occurrence. Base of the middle limestone part of the Kilya 

 Member, Naifa Formation, Naifa Cliff; Beckeri Zone, Upper 

 Kimmeridgian. 



Pachysphinctes sp. indet. 



Material. Many crushed ammonites occur in a shell bed near the 

 top of the Kilya Member in Naifa Cliff and parts of 15 specimens 

 (CA 1019-33) were collected. A few others come from high in the 

 middle limestone part (CAl 174-80) of the same member in Wadi 

 Kilya. Most belong to Pachysphinctes bathyplocus or P. major, but 

 they are not specifically identifiable due to the crushing. They are of 

 Beckeri Zone age. 



Subfamily IDOCERATINAE Spath, 1924 

 Genus IDOCERAS Burckhardt, 1906 



Type species. Ammonites balderus Oppel, 1863, subsequently 

 designated by Spath (1925: 129) (not Roman, 1938). 



Remarks. Spath's designation of Ammonites balderus Oppel as 

 the type species of Idoceras is derived from his declaration 'the true 

 Idoceras of the balderum group' (Spath, 1925: 129). followed 

 immediately by his proposal of the new generic name. Subnebrodites 

 for Ammonites planula Zieten, 1830. By these means the top 

 Oxfordian-Lower Kimmeridgian, evolute, coarse ribbed species of 

 Subnebrodites were separated from the Upper Kimmeridgian 

 (Tethyan), involute, more finely ribbed species of Idoceras, and it is 

 to the latter genus that most of Burckhardt's (1906, 1912) and 

 Imlay's (1939) Mexican forms belong. The separation of the two 

 genera was not accepted by Ziegler (1959), but given the amount of 

 morphological difference, and the stratigraphical separation, it seems 

 to be a useful distinction and was accepted by Callomon ( 198 1 : 149, 

 151). The Yemeni material of Wocera^ described here includes only 

 poor and fragmentary examples from Wadi Hajar (though one is very 

 large), but there are two good specimens from another area 200 km 

 to the west. 



Idoceras ahwarense sp. nov. PI. 8, figs 1, 3; PI. 11, fig. 1 



Types. The holotype is C.71097 and the paratype is C.71098, both 

 from Ghanam al Kuffar ( 13°45"N, 46°4rE), in Wadi Ahwar, 30 km 

 north of Ahwar and 36 km norih of the mouth of Wadi Ahwar on the 

 coast of the Gulf of Aden. 



Other material. SM F. 12164, from the Breadloaf Concretions 

 in the Kilya Member, in the east cliff of Wadi Arus, and CA837, 

 found loose at the base of the Billum Member at the Perisphinctid 

 Cliff in eastern Jebel Billum, having fallen from a higher horizon. 



Diagnosis. A large involute species of Idoceras, reaching at least 

 200 mm diameter in macroconchs. Whorl section compressed, 

 whorl sides nearly flat, rounded umbilical and ventro-lateral edges; 

 narrow, nearly flat venter, with a slight central depression. Ribs are 

 strong throughout growth; primary ribs straight and radial, some 



PLATE 7 



Fig. 1 Pachysphinctes major Spath, base of middle limestone part of Kilya Member (fauna 7), Naifa Cliff, CAIOIO, body-chamber, xO.66. 



Figs 2, 5 Pachysphinctes baliiyplocus (Waagen), base of middle limestone part of Kilya Member (fauna 7), Naifa Cliff. 2a, 2b, CA98 1 . body-chamber, 



xO.69: 5a, 5b, CA985, body-chamber, xO.66. 

 Fig. 3 Peltoceratoides sp. indet. Middle Storm Bed, Madbi Formation (fauna 2), 1 km east of road, south of river crossing at Al Ma'abir, CAl 183. 

 Fig. 4 Idoceras of. balderum (Oppel), Breadloaf concretions. Kilya Member (fauna 7), east cliff, Wadi Arus, CA735, body-chamber. 



