GENUS ORTHOCERAS. 355 



ORTHOCERAS ABNORME (n. s.).' Cu^hi^o W^k 

 PLATE 18 (9), FIG. 10. d^i 1< «> T »t 



Shell large, gently curving and rapidly expanding from the apex : trans- 

 verse section circular ; septa distant, the space of three measured on 

 the convex side nearly equal to the diameter of the outer margin of the 

 larger one measured, their distance gradually increasing with the diame- 

 ter of the shell. Siphuncle central or subcentral, very large, its diame- 

 ter nearly equalling one-half the diameter of the shell, gently constricted 

 at its junction with the septa. 



The siphuncle is partially occupied by a central core, or a replacement 

 of organic deposition, with radiating ramifications which reach the outer 

 walls of the siphon, and are connected with the filling between the septa. 

 Surface characters, form of outer chamber and aperture unknown. 



The peculiar features of this species are the curving form, giving some- 

 what the aspect of Cyrtoceras with the character of septa and siphuncle of 

 Ortkoceras. The distant septa and extremely large siphuncle, with the 

 partial filling observed in numerous specimens, are distinguishing features. 

 It is a large rapidly expanding form, the specimen figured measuring 

 about five inches in length with the outer chamber preserved, having a 

 diameter of two inches and three-tenths, the greatest curvature being in 

 the lower part. 



Other fragments, identified with this species, show the same curvature as 

 the one figured ; while as they increase in size towards the outer chamber, 

 the curvature diminishes, and the extreme portion is probably quite straight. 

 The organic deposition in the centre of the siphuncle has the aspect of a 

 shrunken flexible or sub-elastic tube, with slender tubular ramifications 

 which extend to the exterior walls, and appear as if they might have com- 

 municated to the chamber without, since the slender rami are now continu- 

 ous with the matter filling these chambers. 



This central deposition of matter, with radiating rami, presents 

 characters identical v^^ith those on which the Grenus Actinoceras of 

 Bronn was founded. I have heretofore indicated their irregularity and 

 want of symmetry as an argument against their organic structure. M. 

 Barrande regards these features as due to an organic deposition within 

 the siphuncle. Such an origin would account for their generally charac- 

 teristic features and their absence of structure, which would certainly be 

 indicated in some specimens, had the parts been at all shelly in texture. 

 At least one other species in the same formation preserves a similar feature 

 in the siphuncle. 



Formation and locality. — In limestone of Niagara age, at Racine, 

 Wisconsin. 



