BELEMNITES OF THE LIAS. 



81 



Proportions. The long diameter at the apex of the alveolus being taken at 100, the 

 ventral radius is 42, the dorsal 58, the cross diameter 75 to 84, the axis 220, 250, 280. 

 In an extreme case, a more lengthened specimen than usual, the axis is 360, and there is 

 a further peculiarity to be noticed — a short dorsal groove. 



Phragmoconr. Somewhat remarkable for smoothness on the dorsal and dorso- 

 lateral regions ; for the ellipticity of the cross section, which augments toward the aperture, 

 the axis appearing to be there in the proportion of 100 to 90; and for the small depth 

 of the interseptal spaces, which are about one ninth of the diameter. In the first 

 quarter of an inch from the apex are about 30 septa, in the second quarter 10, in the third 

 quarter 7, in the fourth 5, in the fifth 4. The right and left sides of the phragmocone 

 are straight, the ventral face very httle concave, the dorsal very little convex. 



The ventral portion of the conotheca, half the circumference, is undulated across by 

 the convexity of the interseptal spaces, and finely striated both parallel to the undulations 

 and lengthways. At the boundary between the ventral and lateral spaces these striae 

 turn up suddenly across the band (which is marked by two or three longitudinal straight 

 lines), to join into and form the hyperboHc arcs, which are traced accurately to the 

 asymptote. Along the band here referred to the striation appears complicated, and this 

 probably is what Voltz refers to in his account of B. compressus, var. c, as already noticed. 

 Beyond this the forward-bent arcs undulate the dorsal region, and are crossed by numerous 

 longitudinal striae, which are most distinct on the middle of the back. 



Varieties. In general figure some specimens are nearly straight-sided, others are 

 more convex in the middle of the apicial region, and more rounded toward the apex, 

 which appears to be always blunt and central. The more convex the medio-apicial 

 space, the shorter is the axis in comparison with the diameter. One of my specimens is 

 very compressed, so that the cross section at the apex of the alveolus gives an ellipse of 

 100 to 75. The axis in this case measures 240. 



One of my specimens from Blue Wick, which has the extreme length of axis 

 already referred to, is remarkable for a short dorsal groove, corresponding in situation to 

 that already noticed in B. dorsalis, p. 58, viz. the lower alveolar region. 



Locality. In the Sandy Cap-beds of the Lias at Blue Wick, with Vermicularia 

 compressa {Bean, Phillips, Cullen). In the Sands at Nailsworth {Moore). 



Bemarks. Though I have described these interesting fossils under three specific 

 names, and think it most convenient to do so, it may possibly happen that more complete 

 inquiry may lead me to regard them as varieties of one species, known as B. covipressus, 

 a name very appropriate, but already pre-engaged by Stahl. If this view should finally 

 prevail, a new comprehensive name will be required, and I hope the title of " Voltzii " 



