Vol I. 



Grabau Ordovician Fossils from North China 



(i) 47 



which separate the lateral from the main alveolar cavities, are thue 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. 



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 



Fig. 10. Chihlioceras 



part of this tube in the natural section shown in fig. 13, PL IV, i. nathani. Diagrammatic longi- 

 e. the specimen from sections of which the reconstruction of the tndinai section along line -/. 



in fig. 7, parallel to axis of 



sheath is mainly developed. lateral alveoli. (Notation as 



in fie. 7.). 



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- 

 Fi 11 chihiiocerwi nathani ent ly formed by the wall of the siphuncle. The sections (text 



Diagrammatic longitudinal section figs. 14, 15 an d 16) show this Structure. The length of the 



through the broadest part of the 



median alveolus, along the line g-h. two lateral alveoli may be 'quite different on opposite sides as 



in fig. 7. (Notation as in fig. 7.) ghown j n the specimen figured (p late IT) fig- 13a ). 



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

 All the specimens are aunulated, the annulatiorjs 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 camera were present they were mainly developed on the dorsal and lateral 



