408 REPORT ON THE STATE CABINET. 



middle of the shell a broad and somewhat flattened appearance. 

 The outer chamber gently converges again above the last septum, 

 and is broadly constricted below the aperture, where it is nearly 

 straight for a short distance. Section transversely oval, and a little 

 fliattened on the ventral border in the middle portion of the shell, 

 while in the lower or smaller part it is circular. Septa distant 

 about one-sixth the transverse diameter of the shell, moderately 

 concave, their mai'gins directed slightly upwards on the lateral 

 portions of the shell, giving a broad shallow sinus on the dorsal and 

 ventral sides. Siphuncle small, situated near the dorsal margin. 

 Surface of the shell marked by obscure longitudinal ridges, distant 

 from each other from a sixteenth to a tenth of an inch in different 

 individuals and on different parts of the shell. The impressions 

 of these ridges are distinctly seen on well-preserved casts of the 

 interior. 



This species is readily distinguished from the others of the genus 

 associated with it, by the slight curvature and the greater transverse 

 diameter ; the relative diameters of the specimens figured, in its larger 

 part, being as four to five. The relative distance of the septa may vary 

 somewhat in different individuals, judging from other specimen before 

 me, but not materially affecting characteristic features of the species. 

 The outer two septa, next the chamber of habitation, are often crowded 

 closely together. 



Formation and Locality. — In limestone of the age of the Niagara group, 

 at Racine, Wisconsin. 



Cyrtoceras rigidum. Hall. 



PLATE XVI, FIGS. 3, i, 5. 



Shell small, consisting of about one volution, gradually expanding to a 

 diameter of three-fourths of an inch with a height of two inches. 

 Septa unknown ; outer chamber deep. Siphuncle apparently dorsal. 

 Surface of cast marked by strong obliquely transverse annulations 

 which become gradually stronger from the concave side, and are 

 gently curved backward on the middle of the dorsum. Spaces 

 between the annulations concave, without any evidence of inter- 

 mediate smaller ridges. Exterior surface unknown, the specimen 

 being a cast of the interior. 



