128 



BRITISH BELEMNITES. 



some appear to me quite indistinguishable from their analogues in the older deposit ; they 

 occur of equal magnitude with therm, but not in equal abundance, in the upper part of 

 the Kimmeridge Clay of Shotover Hill, where it was cut through by the railway. One 

 extremely lengthened variety of this Belemnite occurs at Shotover, reminding us of 

 B. spicularis from the shore of Cromarty. 



Besides these is a young depressed Belemnite much like the young B. hastatns of the 

 Oxford Clay ; these occur near Oxford and in Portland Isle. 



Belemnites explanatus, n. s. PI. XXXVI, figs. 94, 95, 96. 



Guard (old). Conoidal, tapering gradually to a rather compressed apex; sides more 

 or less broadly channelled ; ventral aspect flattened and somewhat expanded, becoming 

 concave towards the apex (a few dorsal striae about the apex are sometimes seen). 



Dmensions. Axis about 3 inches ; diameter at the alveolar apex 0"85 inch. 



(Young.) Depressed, smooth, flattened on the ventral aspect, and hollowed, or 

 marked by a narrow groove towards the apex, which is slightly curved ; sides more or less 

 marked by a shallow continuous furrow (a very young form is almost fusiform). 



Dimensions. Axis in the smallest about 1 inch, with a diameter of 2. 5 inch. 



Proportions. Axis in young specimens 400 — 450. The diameters at the alveolar 

 apex 100 from front to back, 115 from side to side. In old specimens the axis is about 

 350, the transverse diameter 107, the dorsal radius 64, the ventral 36. 



Locality. In the upper part of the Kimmeridge Clay of Waterstock, near Thame. 

 Specimens of different ages — young (not middle-aged) and full-grown — have been 

 presented to the Oxford Museum by Mrs. Ash worth. In the upper part of the same 

 clay at Hartwell, near Aylesbury, with Cardiiim incgqinstriatim, Astarte Hartwelliana, 

 and Ammonites biplex. In the Kimmeridge Clay, upper part, where cut through in the 

 railway-tunnel, at Wheatley, near Oxford. 



Observations. On many accounts this form of Belemnite is of interest in the study of 

 the series to which it belongs. On the one hand its resemblance to the older type of 

 B. ahbreviatus {excentricus) of the Oxford Clay and Oolite, and on the other to that of 

 Speeton, in Yorkshire {B. lateralis), is such as to offer a most instructive example for 

 study, in relation to the derivation of successive specific forms by hereditary trans- 

 mission with modification. But this must be considered hereafter. 



