Vol. I. C. C. Yu — Ordovician Cephalopoda of Central China (ii) 61 



The thin septa are rather oblique, sloping back more than 20° and dying away on 

 the convex side. No trace of the siphuncle can be recognized though it seems to be 

 marginal to the convex side. The distances between the septa vary from 8 to 10 mm., 

 where the shell increases in diameter from 15 mm. at the lower to 18 mm. near the upper 

 end. The camerse are entirely occupied by the dark gray lime-mud. The total length 

 of the imperfectly preserved specimen is 108 mm. 



The test is preserved in some places. It is rather thin, showing a thickness of o.i 

 mm. Its surface is apparently smooth. 



Comparison: This specimen is remarkable because of the obliquity of its septa, 

 a feature not seen in any of the p^ceding specimens described. It comes very close in 

 general aspect to Orthqceras durmUm Blake,' from the Lower Llandeilo Limestone of 

 Durness, but in the latter form the rate of tapering is less rapid and the septal distances 

 are shorter than they are in the present form. It may also be separated from Orihoceras 

 baculoides Blake^ by its much*slower tapering and by the apparently eccentric position 

 of its siphuncle, which is central and large in Blake's species. 



Horizon and Locality: At Pei-ting-tze (^^j^jj.) 15 li west of Hu-chia-chi, Chung- 

 hsiang-hsien, where the upper part of the Neichiashan formation is very prominently 

 developed. 



Orthoceras sp. 



Plate IX., Fig. 4. 



Shell straight, cyUndrico-conical and of moderate size. It rapidly tapers 

 backwards to an acute point with an apical angle of 15''. The shell section is elhptical? 

 Test not observed. The siphuncle has been completely rubbed away. 



The septa are thin, direct and numerous. They are 3 mm. distant from each other 

 near the initial part. At the last camera 140 mm. from the apex, the septal interval 

 increases to 9 mm. while the diameter of the shell is 21 mm. The rates of increase are 

 different at the different stages of the conch. Thus, it is rapid at the apical portion, 

 being about i in 4. Above this place 40 mm. from the apex, the shell tapers less rapidly 

 at the rate of about i in 8.5. Near the larger end the tapering is reduced to i in 11. 

 Since the polished section is not a median one, neither the diameter nor the tapering of 

 the specimen can be taken as accurate. 



Remarks: This specimen is characterized by its numerous septa and its variable 

 rate of increase in the different stages of the same individual. Since it is only in part 

 preserved, its characters are insufficiently shown for specific determination. 



1. Blake: British Fossil Cephalopoda, Pt. I, p. 83, pi. Ill, figs. 3, 3a. 



2. Blake: British Fossil Cephalopoda, Pt. I, p. 82, pi. Ill, fig. 2. 



