Fang et al.—Ordovician actinocerid nautiloids from Tibet


P. yaliense has shorter septal necks and denser camerae than other species of Pomphoceras.


Family Wutinoceratidae Shimizu and Obata, 1936 emend Flower, 1968


Genus Wutinoceras Shimizu and Obata, 1936 Type species.—Nybyoceras foerstei Endo, 1930


Wutinoceras cf. W. foerstei (Endo, 1930) Figures 4.2, 5.1–5.3


1930 Nybyoceras foerstei Endo, p. 298, pl. 60, figs. 1a–c.


1932 Nybyoceras foerstei; Endo, p. 79, pl. 25, figs. 3–5, pl. 26, figs. 9, 10.


1933 Nybyoceras foerstei; Teichert, p. 145, pl. 10, figs. 9, 10. 1936 Wutinoceras foerstei; Shimizu and Obata, p. 263.


1957 Nybyoceras foerstei; Yü and Chao, p. 233, pl. 128, figs. 9, 10.


1965 Wutinoceras foerstei; Chao et al., p. 62, pl. 15, figs. 1, 2.


Occurrence.—Lhasai Formation, Darriwilian (Middle Ordovi- cian), near Lhasai village, about 26km east of Xiongmei town, Xainza County, North Tibet.


Description.—Specimen NIGP166261 is a 101mm long frag- ment of an incomplete straight chamber, 65mm in diameter, with conch cross section nearly circular. External shell wall is not preserved. There are 4.8 camerae in the length of the cor- responding conch. The siphuncle is ventral in position. The siphuncle wall consists of long recumbent cyrtochoanitic septal necks and thick connecting rings. The ratios of maximum dia- meter of connecting ring per corresponding conch diameter are about 0.36. SCR ranges from 2.2 to 2.9. The diameter of the septal foramina is 4.6mm. The brims are short, 0.4mm in length, and septal necks are 0.8mm in length. The segments connect with the bottom of the septa on the dorsal side and connect with the top of the septa on the ventral side. Cameral deposits are better developed at the episeptal-mural and hypo- septal positions in the anterior. Siphuncular deposits developed in the central canal. The central canal is probably 2.0mm in diameter. Distal ends of radial canals are nearly horizontal with perispatial deposits.


Material.—An incomplete orthocone cut and polished in med- ian section. NIGP166261.


Remarks.—The external construction, conch shape, and siphuncular characteristics of this specimen are the same as those of the type material of Wutinoceras foerstei (Endo, 1930), but this specimen has the longer septal necks and narrower brim, which may be the result of bad cutting and polishing. Conse- quently, this specimen has been identified as Wutinoceras cf. W. foerstei (Endo, 1930). The species is most similar to Wutinoceras lui Chang, 1959 with regard to the position and construction of the siphuncle, except that the segments of W. lui are not in contact with the top of the septa on the ventral side. W. remotum Chen, 1975 has sparser camerae with 3–3.5 camerae in the length of the corresponding conch and


405


segments are not in contact with the bottom of the septa on the dorsal side. W. moeseini (Thein, 1968) has a subventral siphuncle and longer brims, which differs from W. foerstei. W. robustum (Kobayashi and Matumoto, 1942) from the Darriwllian of Malaysia (Stait and Burrett,1982) was assigned to W. moeseini (see Niko and Sone, 2014) because the SCR of this specimen is approximately 2.0, which is smaller than that of W. foerstei.


Genus Mesowutinoceras Chen, 1976 Type species.—Mesowutinoceras discoides Chen, 1976


Mesowutinoceras giganteum Chen in Chen and Zou, 1984 Figures 4.8, 6.6


1984 Mesowutinoceras giganteum Chen in Chen and Zou, p. 75, text-fig. 24, pl. 5, fig. 3, pl. 6, figs. 13, 14.


Occurrence.—Lhasai Formation, Darriwilian (Middle Ordovi- cian), near Lhasai village, about 26km east of Xiongmei town, Xainza County, North Tibet.


Description.—A large orthoconic fragment is available that is 29mm in length and 20mm in width, with only its dorsal side preserved. Siphuncle is subcentral in position. Siphuncular wall consists of long recumbent cyrtochoanitic septal necks and thick connecting rings. Brims are relatively long and adnation area is broad. Ratios of maximum diameter of connecting ring per corresponding conch diameter are about 0.48; segment is ellipsoid. Cameral deposits weakly developed at episeptal and hyposeptal positions. Siphuncular deposits are filling siphuncle. Central canal is unknown and perpendicular to radial canal. Distal ends of radial canals are anastomosing with peri- spatium near the most inflated portions of connecting rings.


Material.—A large orthocone cut and polished in median section. NIGP166266.


Remarks.—This specimen is very similar to M. giganteum described by Chen in Chen and Zou (1984) from the Ordos area because of its large size, subcentral siphuncle position, ellip- soidal segment, and recumbent septal necks, except that only the dorsal side of the specimen studied herein is preserved. This species differs from the type species M. discoides (Chen, 1976) from the Shandong area in being larger and having awider body size and wider segments.


Family Armenoceratidae Troedsson, 1926 Genus Armenoceras Foerste, 1924


Type species.—Actinoceras hearsti Parks, 1913.


Armenoceras tani (Grabau, 1922) Figures 4.4, 7.5, 7.7, 7.8


1922 Actinoceras tani Grabau, p. 80, pl. 7, figs. 4–7. 1932 Armenoceras hatai Endo, p. 86, pl. 18, figs. 1–3.


1932 Armenoceras tani; Endo, p. 99, pl. 21, fig. 7, pl. 14, fig. 2, pl. 40, fig. 3.


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