FOSSILS OF THE NIAGARA GROUP. 415 



Obthoceras abnorme, n. s. 



PLATE XVIII, FIG. 10; PLATE XXV, FIG. 18. 



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 diameter of the shell. Siphuncle central or subcentral, very 

 large, its diameter nearly equaling one-half the diameter of the 

 shell, greatly 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 Orthoceras. 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 ramifica- 

 tions which extend to the exterior walls, and appear as if they might 

 have communicated to the chamber without, since the slender rami are 

 now continuous with the matter filling these chambers. 



This central deposition of matter, with radiating rami, presents char- 

 acters identical with those on which the Genus 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 characteristic features 

 and their absence of structure, which would certainly be indicated in 



