408
Journal of Paleontology 92(3):398–411
and a narrower siphuncle, which is one-third of the corresponding conch diameter. A. manchurense Kobayashi (1927) from Liaoning area is similar to A. tani except for the large siphuncle size and complex cameral deposits. A. tani shares several characteristics with A. coulingi (Grabau, 1922) from Shandong area (e.g., siphuncle diameter, cameral density, and septal concavity), but A. tani has a subcentral siphuncle and more-depressed siphuncular segments.
Armenoceras teicherti Endo, 1932 Figures 4.6, 7.6, 7.9, 7.10
1932 Armenoceras teicherti Endo, p. 89, pl. 19, fig. 1, pl. 40, fig. 6.
1965 Armenoceras teicherti; Chao et al., p. 67, pl. 16, fig. 8.
Occurrence.—Lhasai Formation, Darriwilian (Middle Ordovi- cian), near Lhasai village, about 26km east of Xiongmei town, Xainza County, North Tibet.
Description.—Specimen NIGP166269 is an almost complete orthoconic phragmocone with circular cross section; it is 67mm in length and 24mm at maximum lateral diameter within 29 camerae. It has a moderate expansion rate, with 11° in apical part. External features and body chamber are not preserved. There are 6, 8, and 10 camerae of corresponding conch diameter in the apical, middle, and adoral parts, respectively. The siphuncle is central in position. The siphuncular wall consists of recumbent cyrtochoanitic septal necks and thin connecting rings. Ratios of maximum diameter of connecting ring to cor- responding lateral conch diameter are about 0.3. Siphuncular segment is discoidal and SCR is 1.8–2.2. Septal necks are 0.5mm in length where brims are 0.8mm and adnation area is broad. Cameral deposits are well developed in episeptal-mural and hyposeptal positions. Siphuncle is filled with endosiphun- cular deposits. The vascular system is unknown.
Material.—A complete orthocone cut and polished in median section. NIGP166269.
Remarks.—Armenoceras tani (Grabau, 1922) is most similar to A. teicherti, but differs in having a depressed circular cross section and a subcentral siphuncle. A. coulingi (Grabau, 1922) has a larger body size than A. teicherti. A. concavum (Endo, 1932) is different from A. teicherti in having higher camerae and smaller SCR.
Armenoceras xizangense new species Figures 4.5, 7.1–7.4
Holotype.—Holotype NIGP166267 (A complete orthocone cut and polished in median section).
Diagnosis.—Large siphuncle, occupying two-thirds of the cor- responding later conch diameter, small SCR of approximately 4.
Occurrence.—Lhasai Formation, Darriwilian (MiddleOrdovician), near Lhasai village, about 26km east of Xiongmei town, Xainza County, North Tibet.
Description.—A large, longiconic orthocone with gradual shell expansion and dorsoventrally depressed cross section is avail- able for study. Phragmocone is 101mm in length and 37.2mm in lateral conch diameter. External wall and body chamber are not preserved. There are 3.6–4.5 camerae per corresponding lateral diameter. Siphuncle is large and subcentral in position. The ratio of lateral diameter to length is 3.5. The siphuncular wall consists of recumbent cyrtochoanitic septal necks and thin connecting rings. Diameters of septal foramina are 7.5 and 9.3mm at lateral diameters of 28.7 and 32.3mm, respectively, where maximum diameters of connecting rings are 16.8 and 21mm. Ratios of maximum diameter of connecting rings to corresponding lateral conch diameter are about 0.67. SCR is 2.2–2.3. Brims are relatively long, 1.7mm in length where septal neck is 0.9mm and adnation area is broad. Cameral deposits well developed in episeptal-mural and hyposeptal positions. Siphuncle is filled with deposits except for the vas- cular system. Radial canals are perpendicular to central canal. Distal ends of radial canals anastomosing with perispatium.
Etymology.—The species name ‘xizangense’ is in reference to the origin of the holotype, Tibet, also called ‘Xizang’ in Chinese.
Remarks.—This new species shares several characteristics with A. teicherti and A. submarginale (Grabau, 1922), for example, the moderate expansion and large siphuncle, but A. teicherti has a circular cross section and A. submarginale has a submarginal siphuncle, which distinguish them from A. xizangense. A. xizangense is most similar to A. magnitubulatum Endo (1932) with its large siphuncle size, but A. xizangense has a wider siphuncle occupying two-thirds of the lateral conch diameter rather than three-fifths as in A. magnitubulatum. A. magnitubulatum also has larger foramina and endocones. A. xizangense has 3.6–4.5 camerae per corresponding lateral diameter, less than the five camerae of A. centrale (Kobayashi and Matumoto, 1942), and A. centrale has a smaller siphuncle, which is less than one-half of the lateral conch.
Family Ormoceratidae Saemann, 1853 Genus Deiroceras Hyatt, 1884
Type species.—Orthoceras python Billings, 1857
Deiroceras globosom Zou and Shen in Chen and Zou, 1984 Figures 4.7, 6.1–6.3
1984 Deiroceras globosum Zou and Shen in Chen and Zou, p. 77, pl. 13, figs. 14, 15.
Occurrence.—Lhasai Formation, Darriwilian (Middle Ordovician), near Lhasai village, about 26km east of Xiongmei town, Xainza County, North Tibet.
Description.—Specimen NIGP166264 is a longiconic ortho- cone with subcircular cross section; it is 31mm in length and 11mm in lateral conch diameter. It has moderate expansion, with 9° angle in apical part. External shell wall and body chamber are not well preserved. There are 3–4 camerae per
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54 |
Page 55 |
Page 56 |
Page 57 |
Page 58 |
Page 59 |
Page 60 |
Page 61 |
Page 62 |
Page 63 |
Page 64 |
Page 65 |
Page 66 |
Page 67 |
Page 68 |
Page 69 |
Page 70 |
Page 71 |
Page 72 |
Page 73 |
Page 74 |
Page 75 |
Page 76 |
Page 77 |
Page 78 |
Page 79 |
Page 80 |
Page 81 |
Page 82 |
Page 83 |
Page 84 |
Page 85 |
Page 86 |
Page 87 |
Page 88 |
Page 89 |
Page 90 |
Page 91 |
Page 92 |
Page 93 |
Page 94 |
Page 95 |
Page 96 |
Page 97 |
Page 98 |
Page 99 |
Page 100 |
Page 101 |
Page 102 |
Page 103 |
Page 104 |
Page 105 |
Page 106 |
Page 107 |
Page 108 |
Page 109 |
Page 110 |
Page 111 |
Page 112 |
Page 113 |
Page 114 |
Page 115 |
Page 116 |
Page 117 |
Page 118 |
Page 119 |
Page 120 |
Page 121 |
Page 122 |
Page 123 |
Page 124 |
Page 125 |
Page 126 |
Page 127 |
Page 128 |
Page 129 |
Page 130 |
Page 131 |
Page 132 |
Page 133 |
Page 134 |
Page 135 |
Page 136 |
Page 137 |
Page 138 |
Page 139 |
Page 140 |
Page 141 |
Page 142 |
Page 143 |
Page 144 |
Page 145 |
Page 146 |
Page 147 |
Page 148 |
Page 149 |
Page 150 |
Page 151 |
Page 152 |
Page 153 |
Page 154 |
Page 155 |
Page 156 |
Page 157 |
Page 158 |
Page 159 |
Page 160 |
Page 161 |
Page 162 |
Page 163 |
Page 164 |
Page 165 |
Page 166 |
Page 167 |
Page 168 |
Page 169 |
Page 170 |
Page 171 |
Page 172 |
Page 173 |
Page 174 |
Page 175 |
Page 176 |
Page 177 |
Page 178 |
Page 179 |
Page 180 |
Page 181 |
Page 182 |
Page 183 |
Page 184 |
Page 185 |
Page 186 |
Page 187 |
Page 188 |
Page 189 |
Page 190 |
Page 191 |
Page 192 |
Page 193 |
Page 194 |
Page 195 |
Page 196 |
Page 197 |
Page 198 |
Page 199 |
Page 200 |
Page 201 |
Page 202 |
Page 203 |
Page 204 |
Page 205 |
Page 206 |
Page 207 |
Page 208 |
Page 209 |
Page 210 |
Page 211 |
Page 212 |
Page 213 |
Page 214 |
Page 215 |
Page 216 |
Page 217 |
Page 218 |
Page 219 |
Page 220
