Vol /. 



Grabau Ordovician Fossils from North China 



(i) 45 



Fig. 2. 



Fig. 3. 



either flat or slightly arched inwards with depressed sides. In its general form and 



character the final endosheath suggests the conotheca of the Belemnite which is 



prolonged forward into the delicate blade-like 



proostracum, or the similar blade of the o- 



dern'cuttle-fish. This blade-like prolongation 



slopes ventrad until it apparently joins the 



wall of the siph uncle on the ventral side. On 



the dorsal side, the blade may have been 



covered by the shell of the siphuncle, but of 



this there is no positive evidence. Indeed 



the sections negative it, although it is of 



course conceivable that the shell was broken 



away before burial. 



The endocone is triple in character. 

 In the genotype the main part has a sub- 

 quadrangular to sub-crescentric cross-section, 

 but appears to taper into a more or less flat- 

 tened conical alveolus towards the apex. On 



Fig. 2. Cnihlioceras nathani. Dorsal view of a mo- 



the ventral Side, Where the Wall of the endo- del of the siphuncle of this species with the compound 

 -, . , ,, i i j ,1 endocone in place. Two thirds natural size. 



cone is prolonged into the blade, the inner Fig 3 chihlioceras na ^ n( . Side view of the same. 



surface is gently convex i. e. arched upward (The annulations of the surface are not represented.) 



(dorsally) with the sides sharply depres- 

 sed (see text fig. 7). The upper surface of 

 this cavity is flat or nearly so, except for 

 the median portion, which is prolonged 

 dorsally into a broad notch or emargina- 

 tion, on either side of which lie the 

 dorso-lateral alveoli. The outer wall of 

 these is rounded, but the inner wall con- 

 sists of two limbs, approximately at right 

 angles to each other, one, the dorso- 

 ventral limb, separating it from the 

 median prolongation of the main cavity, 



Fig. 4. Ot&Uoeenu nathnni. Dorsal view of a model of, ,. ,. ,,..,. 



the final endosheath, whirl, forms the compound endocone of thc other bein g horizontal and dividing 



the siphuncle of this species. TWO thirds natural size. t } ie lateral and main alveolar cavities (see 



Fig. 5. Chililioceras mdhnni. Side view of the model of 

 the endocone shown in fig. 4. Two thirds natural size. text figs. 4 and 7). The position of this 



Fig. 4. 



Fig. 5. 



