BELEMNITES OF THE LIAS. 



63 



Belemnites subaduncatus, Voltz. PL XI, fig. 29. 



Beference. Belemnites subadimcatus, Voltz, ' Obs. sur les Belemn.,' p. 48, pi. iii, 

 fig. 2, 1830. 



B. expansus, Simpson, * Lias Belemnites,' No. 39, p. 46, 1855. 



GuAKD (Young). Cylindroidal, or siibprismatic in the alveolar region, thence tapering 

 in a lanceolate form (so as to be slightly or even distinctly subhastate) by a gentle curve 

 to a very pointed prominent end, which bends a little toward the dorsal side. Three 

 grooves part from near the ungrooved end, two of them dorso-lateral, becoming distinct 

 at a short distance from the end, and gradually vanishing before reaching the alveolar 

 apex ; the third medio-ventral of variable length and distinctness, usually short, but 

 perhaps never quite absent.^ 



(Old.) Few examples are certainly known ; in them the figure is more cylindroidal 

 and more compressed, the termination is less acute and more recurved, so as to resemble, 

 except in greater length, mature individuals of B. vulgaris, from which in youth they are 

 quite distinct. 



Longitudinal sections show the apicial line to be somewhat curved, nearest the ventral 

 face, and three, four, or five times as long as the diameter, according to age. Transverse 

 sections in the young are subquadrangular across the alveolar region, somewhat oval 

 behind it, and trilobed near the end ; in the older specimens this section is decidedly oval. 

 In all examples the axis is excentric, in the young remarkably so ; the dorsal laminae 

 usually thickest over the alveolus, but the contrary also occurs. 



Dimensions. The smallest specimen in my collection is 2 inches long (of which the axis 

 is li), with a diameter of The sizes are traced with certainty to a diameter of finch, with 

 an axis of 1|. Other examples have a longer axis; but, on the whole, it appears, the proportions 

 grow more robust with age. Voltz describes and figures specimens of intermediate magni- 

 tude only. I am uncertain as to the really old forms of the species ; but I believe B. distortm of 

 Simpson, No. 31 (Whitby Museum), to be a good example. On the whole, we may be 

 sure of the identification of the young and middle-aged forms so common on the AVhitby 

 Scars, and so remarkable for their slender shape, very acutely pointed apex, slightly 

 hastate figure (though this varies, and is sometimes only just traceable) by reason of a 

 gentle swelling at about two thirds of the distance from the end toward the alveolar apex. 

 The rather prismatic shape of the alveolar region caused Voltz to call it tetragonal, and 

 sometimes the expression is correct. 



' In my oldest specimen, -J inch in diameter, it is hardly traceable. 



