694 Systematic Paleontology — Upper Devonian 



baso-lateral angles. Air chambers numerous, often somewhat irreguhir 

 in their depth, those near the chamber of habitation sometimes shallower 

 than those preceding. In a portion of the tube, which has a lateral diam- 

 eter of about forty mm. at the smaller extremity, and of forty-six mm. 

 at the base of the chamber of habitation, making about one-quarter of a 

 volution, there are nine air chambers, which vary in depth from six to ten 

 mm., as measured from the summits of the lateral saddles, the shallowest 

 being the last but one. In a smaller individual there are nine chambers 

 in less than half a volution of the septate portion preceding the commence- 

 ment of the chamber of habitation. The septa are strong, especially in the 

 older shells; much thickened on their exterior margins, and strongly 

 imbricating. In their course from their origin on the umbilical margin 

 they curve more or less abruptly backward to a point from one-qiiarter 

 to one-third the diameter of the volution or sometimes even less, where 

 they make an acute return, and, curving forward over the later face of the 

 voliTtion, make a retral bend, which terminates at a point within one- 

 fifth or one-sixth of the width of the volution from the peripheral 

 margin, whence they make an acute turn forward, and pass over the 

 margin of the periphery in an abrupt curve, and descending slightly 

 describe a narrow, acute lobe upon the center of tlie ventrum. This 

 course of the septa includes a narrow acute lobe, near the inner margin 

 of the volution, and thence describes a broad, obliquely semielliptical 

 saddle; a second narrow, elongate acute lobe, near the outer margin, and 

 a narrow obtuse saddle on the periphery, with one side extremely elongate, 

 and tlie other very short. The septum describing the wide lateral saddle 

 extends forward, in its advancing curve, to a distance equal to the great- 

 est depth of nearly two air chambers beyond its origin at the umbilical 

 margin. In its retral curve to the bottom of the acute outer lateral 

 lobe, the distance is equal to the depth of two and a half to three and a 

 half air chambers, measured in their greatest depth. The ventral lobe is 

 short, penetrating the adjacent air chamber about two and one half mm., 

 where the entire depth is nine mm. The suture lines are strongly im- 

 pressed upon the interior cast, and in a simply exfoliate specimen are 

 comparatively very wide, from the thickening of the septa at their 



