262 PROCEEDINGS OF THE ACADEMY OP 



whorl at the aperture, moderately deep, and showing ahout half of each inner 

 turn. Volutions increasing rather gradually in size, very slightly compressed 

 on the dorsal an 1 lateral surfaces, but wiihout the compression imparting any 

 angularity to the dorsolateral and ventral margins, which are rounded ; each 

 concave within for the reception of the inner turns. Septa moderately con- 

 cave, separated by spaces measuring, on trie dorsal or outer side, less than one- 

 third the dorso-ventral diameter of the whorls at the point of measurement, 

 all crossing the sides and dorsum with a b oad backward curve. Siphon 

 scarcely more than its own breadth from the ventral or inner side. Aperture, 

 judging from the section of the whorls, about as wide transversely as its diam- 

 eter, in the direction of the plane of the shell, subquadrilateral, or approach- 

 ing subreniform, in consequence of the sinuosity of the inner side. Suiface 

 unknown. 



Greatest diameter about 4 - 70 inches ; convexity about 2-75 inches ; breadth 

 of umbilicus, l - 45 inches. 



Locality and position. Upper Coal Measures, Lasalle, Illinois. 



Subgenus CRYPTOCERAS, d'Orbigny, 1847. 

 Nautilus (CitYrTOCERAs) cafax, M. & W. 



Shell attaining a moderately large size, subglobose in form. Umbilicus 

 deep, with abruptly sloping walls, one third as wide as the dorso ventral 

 diameter of the outer whorl, and showing each of the inner turns. Whorls 

 about two and a half, increasing rapidly in size, particularly in breadth ; last 

 one so expanded laterally as to be apparently one third to one half wider than 

 its dorso-ventral diameter ; inner ones proportionally narrower. All broadly 

 rounded on the outer side, and more narrowly rounded with a flattened 

 or slightly concave revolving space between the lidge bounding the umbilicus 

 and the middle of each side ; each provided with a narrow, shallow impres- 

 sion along the ventral side for the reception of the inner volutions. Septa 

 separated by spaces which measure, on the outer side, less than one fourth 

 the dorso-ventral diameter of the volution at the point of measurement ; 

 a little arched backwards on the slightly concave inner side of the whorls, 

 and less distinctly so on the narrow revolving flattened space just outside 

 of the umbilicus, after which they cross over the broadly rounded outer 

 side, with a very broad, scarcely perceptible backward curve. Aperture 

 transversely oval, or subelliptic. Outer chamber very capacious, composing 

 less than half of a volution. (Surface unknown). 



Greatest diameter across the disc, about 7 inches ; breadth (transverse di- 

 ameter of the aperture), 6 inches ; dorso-ventral diameter 3-25 inches ; breadth 

 of umbilicus 1 inch. 



The only specimen of this species we have seen is a cast, which shows, 

 along the outer side of the whorls, the appearance of a tube 0'20 inch in di- 

 ameter, extending backwards from each teptem. It is barely possible that 

 this may be a small lobe, but we have scarcely any doubts in regard to its 

 being the siphon, and hence that the species belongs to the group Cryp- 

 toceras. 



Compared with N. dorsalis, Phillips (Geol. Yorks, ii. pi. xviii. fig. 1 and 2), 

 the type of the group Cryptoceras, our shell will be found to differ in its much 

 more broadly rounded dorsum, and much wider mouth, as well as in the pecu- 

 liar revolving flattened space near the umbilical side of the whorls, which im- 

 parts a slight angularity to the margin of the umbilicus, as well as an unde- 

 fined longitudinal ridge, or prominence near the middle of the whorls on each 

 side. 



Locality and position. Charboniere, Missouri. Coal Measures. 



Nautilus (Cryptoceras ?) Leidyi, M. & W. 

 We only know this shell from the non-septate portion, forming about one 



[Dec. 



