Vol I. 



Grabau — Ordovician Fossils from North China 



(I) 47 



which separate the lateral from the main alveolar cavities, are thus double, with the 

 addition of the crystalline, organically deposited lime (stereoplasm) between the two 

 layers. This is diagrammatically represented in text figures 7 and 10, which represent 

 respectively transverse and longitudinal sections through these walls, (for location see 

 text fig. 7), and can be recognized from an inspection of figs. 8 

 and 9, which represent the actual oblique sections through both 

 walls. 



Fig. 10. Chihlioceras 

 nathani. Diagrammatic longi- 

 tudinal section along line e-f. 

 in fig. 7, parallel to axis of 

 lateral alveoli. (Notation as 

 in fig. 7.). 



That an endosiphuncle extends from the base of the main 

 alveolus to the apex of the siphuncle in suggested by the occur- 

 rence of the apical endosiphuncular scar seen on all the specimens, 

 and is further suggested by the appearance of what seems to be a 

 part of this tube in the natural section shown in fig. 13, PI. IV, i. 

 e. the specimen from sections of which the reconstruction of the 

 sheath is mainly developed. 



A consideration of the structure of the final endosheath 

 in the second species (C. chingwangtaoense (Plate II, figs. 13a, b. ) shows very striking 

 differences, but nevertheless a unity of plan. The main or median alveolar cavity has 

 been much reduced, being subtriangular in outline, and only occupying the central 



third of the endosheath. The cavity too is short, though 

 probably prolonged in the endosiphuncle. The dorso-lateral 

 alveolar cavities are deep, and lenticular in section, the inner 

 side being gently concave instead of rectangular. The parti- 

 tions between the cavities are very thick, formed by the 

 bent-over endosheath, with a thick filling of crystalline lime 

 between. The crystalline filling (probably aragonite) has a 

 radial structure where seen in section of the entire siphuncle. 

 The outer wall of the dorso-lateral alveolar cavities was appar- 

 ently formed liy the wall of the siphuncle. The sections (text 

 figs. 14, 15 and 16) show this structure. The length of the 

 two lateral alveoli may be quite different on opposite sides as 

 shown in the specimen figured (Plate II, fig. 13a). 

 In none of the specimens so far obtained has a camerate portion been preserved. 

 All the specimens are annulated, the annulations being essentially of the type seen on the 

 siphuncle of Piloceras. This suggests a camerate structure but does not prove it. The 

 annuli appear to be slightly oblique, converging forward on the ventral side. This 

 suggests, that if camerae were present they were mainly developed on the dorsal and lateral 



Fig. 11. Chihlioceras nathani. 

 Diagrammatic longitudinal section 

 through the broadest part of the 

 median alveolus, along the \iueg-h. 

 in fig. 7. (Notation as in fig. 7.) 



