BELEMNITES OF THE LIAS. 



75 



Belemnites \ ulgaris, Young a.nd £ird. PI. XVI, figs. 40, 41. 



Beference. Belemnites vulgaris. Young and Bird, Yorkshire Coast, p. 25S, pi. xiv, 



fig. 1, 1st ed., 1822 ; 2nd ed., p. 275, pi. xv, fig. 1, 

 1828. 



Simpson, 'Lias Fossils,' No. 23, p. 28, 1855. 

 B. curtus, Simpson, 'Lias Fossils,' No. 24, p. 29, 1849. 

 B. incurvatiis, Quenstedt, ' Cephal.,' p. 418, pi. xxvi, fig. 15. 



Guard. Short, compressed, tapering in a curve to the apex, which in perfect young 

 specimens is acute, but in older examples always somewhat obtuse; lateral furrows 

 distinct for short distance only, ventral furrow present and distinct for a short space in 

 one variety, obsciu'e or almost obsolete in another. 



Transverse sections show the contour to be more variable in different parts of the 

 guard than is usual. At the apex of the alveolus the outline is oval, with more or less 

 flattened sides somewhat inchned to one another, the ventral breadth being less than 

 the dorsal ; the axis excentric. Toward the apex the ventral region increases in ampli- 

 tude, and the contour changes still more toward the apex, there showing three emargi- 

 nations corresponding to the three grooves. This gives on the whole an irregularity to 

 the surface-curves which is less manifest in other species, but is nevertheless often to be 

 recognised in some of them. 



Longitudinal sections show the form to have been little, if at all, changed by growth ; 

 the inner outlines, down to 1 an inch of axis of guard, giving nearly the same proportions 

 as the outer surface of a specimen with four times as long an axis. 



Greatest length observed 6 inches, including the expanded part of the guard ; 

 greatest length of axis, commonly under 2 inches. 



Proportions. Taking the dorso-ventral diameter at the alveolar apex at 100, the 

 ventral radius is about 45, the dorsal about 55, the cross diameter 80, the axis usually 

 200, 225, 250, 275. 



Young. I have found it difficult to trace this species through its younger forms by 

 selection of specimens on the Scars of Whitby and Saltwick. It appears, however, by 

 sections to vary but little with age. In my youngest specimen the axis of the guard is 

 less than 1 inch long, the normal diameter ^ths — proportions which are also found in older 

 specimens. 



Phragmocone. Oblique, a little arched, with an elliptical section ; sides converging 

 19° in the anterior parts, and 22° near the apex. The axis of the phragmocone in two 

 specimens is more than twice as long as the axis of the guard. The diameters are as 95 

 to 100, or in some examples almost equal. The depth of the iuter-septal spaces is com- 

 monly ^th of the long diameter near the apex ; but in one specimen at a more advanced 



