﻿BEITISH FOSSIL CEPHALOPODA. 147 



Orthoceras gregarium, Sowerby, PL XV. figs. 4, 5. 



1839. Orthoceras gregarium, Sowerby in Murchison's ' Silurian Syst.' t. 8, fig. 16, p. 619. 

 1846. „ „ M'Coy, ' Sil. Foss. of Ireland,' p. 8. 



Syn. 1866. Orthoceras novellum, Barrande, 'Syst. Sil. de Boheme,' pi. 218, 396. 



Not 1843. Orthoceras gregarium? Portlock, ' Geol. Bep.' pi. 27, fig. 8. 



Type. — The specimens in the Museum of the Geological Society, which are 

 labelled as having been figured, do not accurately agree with the figure quoted 

 above, which appears to have been made up of several. There is, therefore, no 

 authentic type specimen. In these fragments the section is apparently circular, but, 

 perhaps, not quite so. The rate of increase of the earlier portion is 1 in 7, but not 

 more than 1 in 12 near the aperture. The body-chamber is at least \\ times its basal 

 diameter, but the aperture is not reached. The shell is thin and smooth, except a 

 few undulating lines on the body-chamber. The septa are direct, and distant about 

 \ the diameter apart, but more remote in youth ; their convexity is \ the diameter. 

 The siphuncle is minute and central. Several specimens occur associated in the 

 same stone. They are from Lower Ludlow rock, Ludlow. 



General Description. — The majority of specimens referred to this species have 

 unequal diameters, but this is in some degree due to pressure, as others are nearly 

 circular or subquadrate. In some, from Lower Silurian beds, there is a depression 

 along one side, which makes the section somewhat cordate, like 0. semipartitum. 

 In other examples of the same age there is a slight curvature observable. The 

 measured rate of increase lies between 1 in 7 and 1 in 9, but is exceptionally less. 

 The longest body-chamber seen is If inches, which is twice the corresponding basal 

 diameter, and yet no line of aperture is reached. The shell in the Lower Silurian 

 examples is thick. Most appear smooth, but several show undulating lines of 

 growth. The septa are mostly direct, but most of the older group show a slight 

 obliquity, amounting to 3° at most ; they are distant from \ to ^ the diameter, and 

 have a convexity of -g- to \ the same. One example from Kildare shows that form 

 of normal line which is produced by the tube-like depression on the inner surface 

 of the septal chambers. The siphuncle is minute and central. The greatest 

 diameter seen is 10 lines, and the greatest length 4 inches. The members of this 

 species appear to have lived in groups, as indicated by Sowerby's name, as several 

 are often found in one slab. 



Relations. — There appears to be scarcely sufficient reason for separating the 

 Lower from the Upper Silurian examples, although the former alone exhibit the 

 curvature, the thickness of shell, and slight obliquity of septa noted above. The 

 central siphuncle separates them from the 0. subgregarium of M'Coy. Salter appa- 

 rently considered the Lower Silurian forms as identical with his 0. audax, but, as 



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