﻿BRITISH FOSSIL CEPHALOPODA. 175 



Cyrtoceras contrarium, Barrande, PI. XIX. figs. 9, 10. 



1866. Cyrtoceras quasi-rectum, var. contraria, Barrande, 'Syst. Silur. de Boheme,' vol. ii. 



pi. 146, p. 586. 



Type. — The normal form named quasi-rectum (described by Barrande on p. 669 

 and figured in pi. 160, figs. 1-9, and pi. 163) has a section which varies from circular 

 to elliptic, with the axes in the ratio of 8 to 7 ; but the variety contraria has them 

 as 7 to 6. The curvature, which is in the plane of the long axes, is zero on one side 

 and very slight on the other, only manifest indeed towards the apex in the variety 

 contraria. The increase of the diameter is variable, at first 1 in 3 and later 1 in 7 or 9. 

 The ornaments are strong rib-like undulations, tending to imbricate upwards, low, 

 round, and ill-defined, at a variable distance apart, much closer at first and becoming 

 more irregular at last, the mean distance being about ^ * ne l° n g diameter, nearly 

 straight on the type and coming to a very slight sinus on the concave side, but in 

 the var. contraria sloping backwards to the convex side. These ribs are covered by 

 from 5 to 8 parallel riblets, giving them their ill-defined character. The length of 

 the body-chamber is about equal to the diameter at its base. The outline of the 

 aperture, which is not at all contracted, is as transverse as the ribs are, and has a 

 slight sinus on the same side as the siphuncle. The septa have a convexity of -^ of 

 the long diameter, and are distant ^ to -^ of the same. The sutures are nearly 

 straight, and, though not always parallel to the ribs, are yet oblique in the same 

 direction and nearly to the same amount as they are. The siphuncle is said to 

 be cylindroid and not nummuloid, its longitudinal axis being greater than its 

 transverse, but it is inflated between septa and is therefore bulbous. Moreover, 

 while the normal form is ranged among those with cylindroid siphuncle, the variety 

 contraria is classed with the species having a nummuloid siphuncle. It is situated 

 in the normal form near the concave margin, but in the variety contraria near the 

 convex margin, whence the name of the variety. Length, 2f inches ; greatest 

 diameter, 1^ inches. From stage E in the lower part of the Upper Silurian. 



General Description. — The British forms are very characteristic fossils, and 

 apparently abundant in one locality. The section is a compressed oval, whose 

 length is due in all probability to pressure, as several are changed in their shape 

 from that cause ; the ratios of the diameters in this way vary from 1 1 to 6 to that 

 of 12 to 9. The curvature on the convex side is almost confined to the earlier 

 portion, and the usually concave side is flat, except near the apex, where it also is 

 convex. Thus the rate of the increase of the diameter is variable, at first 1 in 6, 

 but later becoming almost zero. The surface is ornamented by undulating, rough 

 ribs, with parallel riblets of irregular character, narrow towards the apex, then 

 becoming stronger, and again diminishing towards the aperture ; they slope a 



