Fang et al.—Ordovician actinocerid nautiloids from Tibet
corresponding lateral conch diameter. The concavity is nearly equal to the length of one camera. Siphuncle is subcentral in position. Ratios of maximum diameters of connecting rings to corresponding lateral conch diameter are 0.26. Siphuncular wall consists of short, recumbent cyrtochoanitic septal necks and thick connecting rings. Septal foramina are 0.7–0.9mm. Septal necks are comparatively short and brims are weakly recumbent. Connecting rings are weakly inflated in the anterior part of each siphuncle segment. Siphuncular segment is gently rounded, with SCR approximately 0.8. Cameral deposits well developed as episeptal-mural and hyposeptal deposits in ventral side and as episeptal deposits on dorsal side. Siphuncular deposits well developed. Central canal is straight and weakly subdorsal; not- ches developed in the endosiphuncular deposits toward the posterior of the upper of the siphuncular segments, indicating the radial canals are better developed adoral to the septal for- amina rather than apically.
Material.—A longiconic orthocone cut and polished in median section. NIGP166264.
Remarks.—According to Teichert et al. (1964) and Dzik (1984), Deiroceras is most similar to Ormoceras except for the siphuncular segment shape, and Deiroceras has elongate con- necting rings that are longer than wide. This specimen has several similarities with the type
specimen described by Zou and Shen (in Chen and Zou, 1984) from Inner Mongolia , such as 3–4 camerae per corresponding lateral conch diameter, the concavity equal to one camerae length, and moderate siphuncle size.
Family Discoactinoceratidae Kobayashi, 1978 Genus Discoactinoceras Kobayashi, 1927
Type species.—Discoactinoceras multiplexum Kobayashi, 1927
Discoactinoceras cf. D. multiplexum Kobayashi, 1927 Figures 4.9, 6.4, 6.5
1927 Discoactinoceras multiplexum Kobayashi, p. 202, pl. 22, figs. 7a, b.
1931 Discoactinoceras multiplexum; Kobayashi, p. 56, pl. 4, figs. 1a, b.
1965 Discoactinoceras multiplexum; Chao et al., p. 61, pl. 14, figs. 3–6.
1978 Discoactinoceras multiplexum; Kobayashi, p. 226, text- figs. 1, 2, pl. 31, figs. 1–5.
1981 Discoactinoceras multiplexum Kobayashi in Zou, p. 358, pl. 2, fig. 7.
Occurrence.—Lhasai Formation, Darriwilian (Middle Ordovi- cian), near Lhasai village, about 26km east of Xiongmei town, Xainza County, North Tibet.
Description.—The siphuncle is preserved and the orthoconic phragmocone with circular cross section is 21mm in length. Siphuncle is large and central in position. Siphuncular foramina are broad. Siphuncle segments are about four times as wide as
409
long. Septal necks are short and brims are short. Siphuncle is filled with deposits, and vascular systems are unknown.
Material.—An orthoconic fragment cut and polished in median section. NIGP166265.
Remarks.—Kobayashi (1978) proposed a new family Discoactinoceratidae because the genus Discoactinoceras developed intrasiphuncular stereoplasmic deposits and recum- bent cyrtochoanitic septal necks. This specimen (NIGP166265) is most similar to the specimen described by Zou (1981) from the Inner Mongolia area in its body size and large siphuncle.
Acknowledgments
We are indebted to C. Li (Jilin University) for providing several specimens and stratigraphic information. We are grate- ful to D. Evans and an anonymous reviewer for their con- structive suggestions of the manuscript. Financial support from the Chinese Academy of Sciences (XDPB05, XDB10010100), the National Natural Science Foundation of China (grant No. 41290260, 41521061), and the Ministry of Science and Technology of China (2013FY111000) is acknowledged. This is a contribution to the IGCP 653 project (‘The Onset of Great Ordovician Biodiversification Event’).
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