324 Systematic Paleontology — Middle Devoxiax 



transverse diameter of 20 mm. The later faces curve rapidly in young 

 shells, and in older ones are very gently eundng toward the periphery, 

 which is abruptly rounded. 



" Volutions about four, embraced within the outer one, having the 

 umbilicus closed exteriorly, and with a slight depression, which does not 

 expose the inner whorls; somewhat gradually expanding in the young 

 shell and more rapidly in the older ones. Transverse section, semi- 

 elliptical mth the base, deeply concave from the inclosed preceding volu- 

 tion. The increase in width of the last volution, in young, well-formed 

 shells, is from 8 to 15 mm. In a large form it is from 14 to 28 mm., and 

 in a specimen of 65 mm. in its greatest diameter, the increase in the last 

 volution is from 25 to 40 mm., while the largest specimen measured, which 

 is much compressed and somewhat distorted, gives 32 and 60 mm. as 

 the diameters of the base of the last volution and that at the aperture. 



" Chamber of habitation very large, having an area four times as great, 

 and a capacity eight or ten times as great as all the air-chambers. Aper- 

 ture, in old shells, elongate-seniielliptical, with the base deeply indented 

 by the embraced volution; toward the base the margin is laterally ex- 

 panded, and distinctly auriciilated at the baso-lateral angles; thence curv- 

 ing rapidly forward, it is again depressed anterior to the middle of the 

 lateral face, forming a shallow sinus ; whence it advances on the ventro- 

 lateral portions, and, abruptly curving backward, forms a deep sinus on 

 the periphery. Air-chambers very shallow near the axis of the shell, 

 continuing nearly parallel, and then becoming rapidly expanded toward 

 the periphery, gradually increasing in depth with the increasing size of 

 the volutions. 



" The septa, on each side, are closely arranged on the inner half of the 

 volution, and first curve gently fonvard, and thence abruptly recurving 

 on the middle of the lateral face, continue with a more gentle retral bend 

 to the periphery, where they are more than thi-ee or four times as distant 

 as at their origin. In their passage, they describe a broad and deep lateral 

 lobe, which occupies the outer half of the volution ; with a very shallow and 

 scarcely marked saddle upon the peripheral border, and a very short and 

 narrow, ventral lobe. In young shells the sides of the ventral lobe are 



