94 



M.K. HOWARTH 



wiry primary ribs, 12 per half whorl at 1 19 mm diameter, bifurcate 

 irregularly at or just ventral of the middle of the side of the whorl; 

 other secondaries are intercalated, so that 27 secondaries correspond 

 to the 1 2 primary ribs. The primary ribs Eire raised but not tuberculate 

 at the point of bifurcation. The secondaries pass over the venter with 

 no forwards projection, and are interrupted or diminish considerably 

 in strength along the mid-ventral line. There are no umbilical, 

 ventro-lateral or ventral tubercles. The second specimen is a short 

 fragment of a body-chamber, and with a whorl height and width of 

 approximately 40 mm and 33 mm respectively (though the dorsal 

 part of the whorl is missing) it is considerably larger than the first 

 specimen. It has more closely spaced ribs than the latter, and so is 

 unlikely to be a larger part of that specimen. 



Remarks. These Yemen examples appear to be the largest 

 Riasanites yet found, except for Grigorieva's (1938: 94, pi. 1, fig. 2) 

 R. rjasanensis var. maikopensis from the Caucasus, which is based 

 on a 138 mm diameter specimen that has more primary ribs (ca.20 

 per half whorl at 135 mm diameter) than the typical form of R. 

 rjasanensis. The Yemen specimen is a close match for the larger 

 specimens figured by Nikitin (1888), Bogoslovsky (1897) and 

 Sazonova (1977), especially in its characteristically narrow, flat 

 venter with the mid- ventral interruption of the ribs, so there can be 

 little doubt that it is conspecific with them. R. swistowianus (Nikitin, 

 1888: 93, pi. 1, figs 5-8) is more evolute, more depressed and has 

 lateral tubercles at the point of bifurcation of the ribs, while R. 

 subrjasanensis (Nikitin, 1888: 93, pi. 1, fig. 4; Bogoslovsky, 1897: 

 87, pi. 5, fig. 6) is more involute, more compressed, has higher 

 whorls and more ribs. Tauricoceras Kvantaliani & Lysenko (1979) 

 from the Berriasian of the Crimea is a synonym of Riasanites, and its 

 type species, T. crassicostatum is so similar to the figured material of 

 R. rjasanensis, that the two might even be conspecific. Other species 

 of Tauricoceras described by Druschitsefa/. (1984) and Kvantaliani 

 (1989: 15-20, pis 4-12) are based on small specimens that are also 

 very similar to Riasanites rjasanensis. 



The two figured specimens from the Upper Tithonian of Argen- 

 tina, Riasanites rjasanensoides Krantz (1926: 441, pi. 17, figs 1, 2; 

 1928: 25, pi. 4, fig. 7), and R. aff. swistowianus (Nikitin) (Krantz, 

 1928: 27, pi. 4, fig. 8) are similar and close to the Russian examples, 

 and Leanza (1945: 40) described a single example of R. 

 rjasanensoides from the Upper Tithonian, Corongoceras alternans 

 Zone, in Mendoza. 



Riasanites rjasanensis is somewhat similar to Argentiniceras 

 mintaqi sp. nov., which occurs in the Berriasian in the Mintaq 

 Member at the Mintaq Salt Dome, but Riasanites differs in having 

 less quadrate whorls, with convergent whorl sides and a narrower 

 venter, a better developed and more persistent mid-ventral interrup- 

 tion of the ribs, and generally more widely spaced ribs, that are never 

 fine and multi-branched as in some species of Argentiniceras (eg. A. 

 mutatum (Steuer)). Riasanites does not develop lateral or umbilical 

 tubercles to the same extent as in some species of Argentiniceras. 



Occurrence. Lower part of Arus Member, Hajar Formation, 

 Wadi Arus; Microcanthum Zone, Upper Tithonian. 



Genus BLANFORDICERAS Cossmann, 1907 



Type SPECIES. Ammonites wallichi Gray, 1832. 



Synonyms. Blandfordiceras Spath, 1924a, objective synonym; 

 Pseudoblanfordia Spath, 1925 (type species, Hoplites australis 

 Burckhardt, 1903). 



Blanfordiceras wallichi (Gray, 1 832) 



PI. 21, fig. 5 



1832 Ammonites wallichi Gray: pi. 1 10, fig. 3. 



1863 Ammonites wallichi Gray; Blanford: 84, pi. 15, fig. 1; pi. 



19, fig. 1. 

 1904 Hoplites wallichi (Gray); Boehm; 31, pi. 3, fig. 4; pi. 4, figs 



l-4;pl.5, fig. 1; text-figs 7-9. 

 1910 Hoplites (Blanfordiceras) wallichi (Gray); Uhlig: 186, pi. 



29;pl. 30, fig. 1; pi. 31, figs 1,2. 

 1939 Blanfordiceras aff. wallichi (Gray); Spath: 43, pi. 4, fig. 6; 



pi. 5, figs 1,9, 10. 

 1960 Blanfordiceras cf. wallichi Uhlig; Collignon: pi. 166, fig. 



679. 

 197 1 Blanfordiceras cf. wallichi Uhlig; Mouterde: 157, pi. 2, fig. 



3. 

 1996 Blanfordiceras wallichi (Gray); Wright: 50, fig. 38-1 . 



Material. Three specimens: CA969-70 from 4 m above the base 

 of the Mintaq Member, eastern Jebel Billum; CA785 from the 

 microbialite boulders in the Arus Member in the east cliff of Wadi 

 Arus. 



Description. CA969 is a well-preserved fragment about one- 

 third of a whorl long and approximately 110 mm diameter at the 

 larger end; there are apparently no septa, so it is probably part of a 

 body-chamber. The whorls are evolute. with a wide umbilicus, a 

 subquadrate whorl section, rounded whorl sides and a flat venter. 

 Strong, straight ribs mostly bifurcate at or just ventral of the middle 

 of the whorl side, but a few ribs remain single; they are projected 

 slightly on the venter, where they end at tubercles bordering a mid- 

 ventral smooth band. CA970 is in a block of hard porcellanous 

 limestone from which it cannot be extracted and shows the charac- 

 teristic cross-section of a Blanfordiceras at about 100 mm diameter. 

 CA785 is only a mould of part of a venter at approximately 100 mm 

 diameter, but it shows the characteristic rows of ventral tubercles 

 bordering a mid- ventral smooth band or slight depression. 



Remarks. Blanfordiceras is more evolute and has more quadrate 

 whorls than Berriasella. and the ribs on the venter end in small 

 tubercles bordering the mid-ventral smooth band or groove. The 

 Yemeni specimens are close to the holotype of the type species 

 (newly figured by Wright, 1996: fig. 38-1), whilemany of the other 

 forms from the Spin Shales described by Uhlig (1910: 186-203) 

 have depressed, massive whorls and tubercles. Eight specimens 

 from Madagascar described by Collignon (1960: pis 166, 167, figs 

 679-686) include several that are very similar to the Yemeni speci- 

 men. Biostratigraphical association with Kossmatia, Paraboliceras 

 and Virgatosphinctes, and proof that Blanfordiceras is Upper 

 Tithonian in age, was obtained by Mouterde (1971: 157) in Nepal. 



Occurrence. Arus and Mintaq Members, Hajar Formafion. Wadi 

 Arus and eastern Jebel Billum; Microcanthum and Durangites Zones, 

 Upper Tithonian. 



Genus MALBOSICERAS Grigorieva, 1938 



Type species. Ammonites malbosi Pictet, 1867. 



Synonyms. Pomeliceras Gngontva, 1938,«o«Hoedemaker, 1981 

 (type species. Ammonites breveti Pomel, 1889); Mazenoticeras 

 Nikolov, 1966 (type species, Berriasella broussi Mazenot, 1939); 

 IRetowskiceras Nikolov, 1966 (type species, Perisphinctes 

 andrussovi'R&io^ski, \'S,92>).ChapericerasWotdemaktx, 1981 (type 

 species. Ammonites chaperi Pictet, 1868). 



Remarks. Species of Malbosiceras are Berriasella-hke ammo- 

 nites that develop mid-lateral and umbilical tubercles on periodic 



