Maryland Geological Survey 325 



nearly parallel, and extend about one-third of the distance across the 

 adjacent air-chamber. The width of the saddle is equal to the width of the 

 lobe, and the depth of the lobe is equal to one and a half or two air- 

 chambers. The sutures are distinctly marked upon the exterior of the 

 cast by the thickened margins of the septa, which are imbricating toward 

 the outer chamber, and leave a strong groove Avhen weathered. 



" Siphuncle very small, close to the ventral side, and nearly coincident 

 Avith the walls of the peripheral lobe. In one specimen, having its greatest 

 lateral diameter of 28 mm., the width of the volution at the last septum 

 is 7 mm., and the diameter of the siphuncle about 1 mm. on the convex 

 side of the septum. In a specimen exposing the siphuncle between several 

 of the septa, its form is evidently subconical, narrower at the concave face, 

 and wider at the convex face of the septum. 



" The test is very thin, in young specimens being not more than .2 mm., 

 and in older ones not more than .5 mm. The older shells are always 

 extremely compressed. The surface is marked by fine, closely arranged 

 striae, which at intervals are raised in fascicles, giving an undulated aspect, 

 which is often more conspicuous in young than older shells. The striae 

 become more conspicuous as they pass from the lateral faces over the 

 borders of the periphery. There are other fine, gently undulating, concen- 

 tric striae, which are confined to a small area around the umbilicus, giving 

 a delicately ornamented surface. Some of the specimens show a pitted or 

 punctated surface, of similar character to tliat observed on the shell of 

 the recent Nautilus beyond the covering of the mantle. The principal 

 striae make an abrupt retral bend upon the ventro-lateral margins, and 

 describe a deep sinus upon the periphery. 



" The internal cast, in a large proportion of the specimens, is essentially 

 smooth, being marked only by impressions of the striae of growth, and in a 

 few examples by gentle undulations, which indicate the course of the 

 stronger fascicles of the external striae. The marks of the undulating 

 concentric striae around the umbilicus are rarely preserved in the casts of 

 the interior, but the punctate marking is more frequently seen. Small 

 individuals of this species have a lateral diameter of 10 mm., with a 

 transverse diameter of 6 mm. The largest specimen measured, which is 



