Vol I. 



Grabau Ordorician Fossils from North China 



(i) 49 



central specimen shown in Plate V) it has reached a diameter of 30 mm. From this 



point the expansion is regular, until at about the point near the apex of the median 



(Midoeone, about 40 mm. farther (or 56 mm. from the apex of the siphuncle) the lateral 



diameter is 40 mm. This gives a rate of tapering of 1 mm. in 4. The earlier portion is 



regularly rounded, while the part occupied by the endocone is 



somewhat flattened on the ventral side. The endocone occupies 



something more than one half the length of the shell, inclusive of 



the anterior blade. The apical portion in the center is rounded 



dorsally and flat or gently concave on the ventral side, its section 



thus being semi-circular or compressed suboval, with the ventral 



side curved to a greater radius. Proceeding forward, the concave 



central portion narrows and flattens, while the sides of the blade 



become strongly and sharply depressed, until near the anterior 



portion of the blade they form less than a right angle with the 



side. The aperture of the endocone, i. e. the edge formed by its 



meeting with the shell of the siphuncle, is oblique to the axis of 



the siphuncle, the most projecting portion being the center of the 



blade (see the restoration, text figs. 2 and 3). Lateral alveoli of 



the endocone shorter than the main cavity. In the specimen 



shown in the center of Plate V the alveoli are not seen, but in 



Fig. 13. Chihlioceras 



the somewhat crushed right hand specimen of that group, they between median plane and 



. ,-, /T AN lateral man-in , along line 



are recognizable (Plate I fig. 10), being displaced somewhat to k . t in a g< 7. pag ging 

 one side. The inner walls of these lateral alveoli form approxi- through lateral part of me- 



dian, and one lateral alveo- 



mately a right angle, and consist of the reduplication of the ius( Notation as in fig. 7.) 

 endosheath with crystalline calcium carbonate filling between. 



The outer wall of the lateral alveoli is convex and between it and the wall of the 

 siphuncle, there is a thick layer of crystalline lime (organic deposit) which decreases 

 wedge-like towards the rim of the endocone. (Text figure 10). 



These lateral alveoli of the endocone hold a position above the base of the main 

 emloconic cavity, so that there are distinct lateral chambers proceeding from this main 

 median chamber, and in position ventral to the lateral alveoli. This is clearly shown by 

 the sections (Plate II figs. 1 1 and 12) and is represented in the model of the endosheath 

 illustrated in text fig. 4. The endosheath itself (i. e. the wall of the endocone) has a 

 thickness of half a millimeter or less, but because of the filling of crystalline lime between 

 tins reduplicated portions, which form the lateral and median alveoli, the thickness of 

 the compound wall separating these alveoli may be from 2.5 to 6 mm. (See the sections 

 of these walls in figs. 1 1 and 12 Plate II). 



