74 



BRITISH BELEMNITES. 



ventral aspect rather narrower than the dorsal (adult) ; no recurvation of the apex, which 

 is divided by the grooves. 



Sections across the alveolar cavity show the slightly oval alveolar section within the 

 oval or elliptical outline of the guard — the oval outline being caused by the contraction of 

 the ventral region ; sides flattened. 



Dimensions. Length of guard before it shows any alveolar expansion, 2^ inches, of 

 which the axis is about li inch ; the diameter from back to front being in the largest 

 specimen -^ths inch. 



Proportions. The dorso-ventral diameter at the apex of alveolus being taken at 100, 

 the cross diameter is 85, the ventral radius about 40, the dorsal about 60, the axis 250 

 (in the larger variety 300 to 400). 



Locality. Upper Lias Clays at Eydon (one), and Badby (three), near Banbury 

 {Stuttard). 



Observations. The affinity of this tripartite fossil to that so frequent at Whitby 

 (i?. vulgaris) is obvious ; while its parallel sides and greater compression offer analogies to 

 the less obtuse examples of B. digitalis, like Quenstedt, pi. xxvi, fig. 11, and B. incurvatus, 

 fig. 15, on the same plate. 



As in B. vulgaris, so in this, the axis of the guard varies in length in proportion to the 

 diameter. There is a series of more slender forms in Mr. Stuttard's collection from 

 Eydon and Badby, in which the axis measures 320, 350, 400, no other difference being 

 evident; e.g. (Nos. 50, 51, 58), Eig. 38^ shows one of the shortest of these slender 

 forms, with ventral groove longer and more distinct than usual. 



Young specimens (Nos. 53 and 63) from Eydon and Badby have the axis remarkably 

 excentric, in the proportion 28 to 72, the interior nearly circular, sides somewhat chan- 

 nelled, termination acute, ventral sulcus distinct, the others variable ; in one, dorsal plaits. 

 The axis exceeds 600. They resemble almost exactly some young cyHndroid varieties of 

 B. subaduncatus, with the same excentricity. The older specimens referred to also corre- 

 spond very much with the older cylindroid specimens of B. subaduncatus. Thus they 

 constitute a parallel series; and taking the whole into account, we have this general 

 comparison of these tripartite forms : 



Whitby, more or less recurved : 



Short form, B. vulgaris, PL XVI, fig. 40. 

 Long form, B. vulgaris, PI. XVI, fig. 41. 



B. subaduncatus, PL XI, fig. 29. 



Banbury, little or not at all recurved : 



Short form, B. regularis, PL XV, fig. 38 a. 

 Long form „ „ fig. 38 b. 



