260 W. H. SHKKZEB THE GENUS CHONOPHYLLTJM. 



and Haime the septa are stated to be of equal length ; but in all farms 

 examined we have found them alternating, of twO orders, those of the 

 second order terminating as they reach the central pit, those of the first 

 extending to the center. These primaries may remain straight or become 

 more or less twisted, but not so as to form a columella. Viewed from the 

 calyx the direction of the twist is left-handed, i. <■., contrary to the hands 

 of a watch. 



Near the base and along the central vertical axis the septa present the 

 form of vertical lamella?, apparently similar to the ordinary lamellar 

 septa: but as they pass outward from the center and upward from the 

 base to the outer calyx margin they gradually thicken until in some of 

 the largest forms of C. magnificum they are 5 mm across at the periphery 

 of the calyx. In the calyx about the pit the septa appear thin and sharp, 

 but pass outward as gradually broadening convex hands, separated in 

 the outer area by very narrow grooves which mark the position of the 

 interseptal cavities. If the outer edges of these septa are examined in 

 decorticated specimens, or if vertical sections are prepared through this 

 outer area, each septum is seen to lie made up of a series of delicate. 

 regularly curved, superposed membraniform layers with their convexities 

 upward (figure 2). It was these spaces which Billings mistook for the 

 interseptal spaces * as first pointed out by Dr. Rominger.r The regularity, 

 distance apart, and thickness of these layers are subject to some variation 

 in the different species. They are most beautifully and typically devel- 

 oped in C magnificum, where they average about 5 or (5 to the mm, hut 

 range from 3 to 12 to the mm. In this same species their average thick- 

 ness in several thin sections was found to he about ^ mm. the thickest 

 being T \j and the thinnest ones observed -V mm. 



Along the medial plane of each septum these layers are approximately 

 horizontal for a short distance, curve gradually downward toward the 

 sides, and finally are sharply deflected, fusing with one another along 

 their edges to form the side faces of the septum. Occasionally a layer, or 

 a series of layers, unites directly with those just beneath before reaching 

 the side, and thus takes no part, for some distance at least, in the actual 

 formation of the septal faces. When the septum has become too narrow, 

 an upper layer may send down its edges upon each side completely en- 

 wrapping as many as 12 or 15 older ones, thus suggesting their method of 

 growth. In general, these layers are not continuous from one septum to 

 its neighbor, hut each septum is made up of an independent series. 

 Occasionally they pass completely across for a short distance, arching up- 

 ward in the interseptal cavity and assisting in the formation of the vesi- 



*Can. Jour., new series, vol. \. I860, p. 265. 

 fGeol. Siirv. of Mich., vol iii. 1S76, |>t. ii. p. 115. 



