NIAGARA GKOUP. 



293 



660. 37. ORTHOCERAS UNDULATUM. 



PL. LXIV. Fig. 1 a -f; and Pl. LXV. Fig. 3. 



Compare Orthoceras annulatum, Sowerby, Min. Conchology, Vol. ii, 1818, p. 77, t. 133. 



" — — HisiNGER, Anteckn. v, tab. iv, fig. 6. 



« — — Id. Vet. Acad. Handl. 1826, t. vii, fig. 8. 



" — — Id. Petref Suecica, 1837, p. 28, t. x, f. 2. 



Not O. undulatum, Sowerby. 

 " 0. defrancii, Teoost. 



Cylindrical, very gradually tapering ; siphuncle subcentral ; annulated by strong rounded 

 annulations, which are distant from each other one-fourth to one-fifth the diameter j surface of 

 the shell marked by undulating and more or less imbricating laminae. 



In a single imperfect specimen, where the siphuncle is visible, it appears to be a little 

 eccentric, but this appearance may be due to pressure. The specimens obtained have been 

 usually in the soft shale, and completely flattened, so that septa and siphuncle are obliterated. 

 The shell is frequently more or less abraded or dissolved, and the characters of the surface 

 are variable. Sometimes the shell is entirely removed, and the annulations alone mark the sur- 

 face. A comparison with Swedish specimens leaves no doubt of the identity of our species with 

 0. undulatum of Hisinger ; and the figures in Murchison's Sil. System, from the Wenlock 

 formation, are evidently identical with ours. It is extremely doubtful, however, if 0. annulatum 

 of Min. Conchology is the same species, particularly since all the evidence we have of its 

 geological position would go to show that it belongs to a later period, and we have in our 

 guccessive formations several annulated species of Orthoceratites which are not very unlike that 

 one. Under these circumstances I prefer to adopt the name of Hisinger, until it be demonstrated 

 that the two are identical, when the name given by Mr. Sowerby will have precedence. 



Fig. 1 a. A fragment of this species on a slab of shale : the sharpness of the laminae has been 

 much reduced either by maceration or solution. 



Fig. 1 b. Another fragment where the laminee are nearly obsolete. 



Fig. 1 c. A fragment replaced by gypsum, and preserving only the annulations. The substitution 

 of the sulphate of lime has doubtless taken place from the solution of the original 

 shell and the calcareous interior, and we have merely a cast preserved. 



Fig. \ d. k fragment near the smaller extremity, showing the lamince but partially preserved. 

 The very gradual diminution shows the species to have been extremely elongated ; 

 and I have seen fragments of a single individual, more than two feet in length, and 

 imperfect at both extremities. 



Fig. 1 e. Transverse section of an imperfect and somewhat distorted specimen, showing the 

 position of the siphuncle. 



Fig. 1 /. Vertical section showing the convexity of a septum. 



Plate Ixv, fig. 3. A fragment of this species from near the aperture. The shell is nearly all re- 

 moved by maceration, and the interior is mostly covered by the little coral Sagenella. 



