BELEMNITES OF THE OXEORD CLAY. 



119 



B. Ownnii, Qiienstedt, ' Ccplialop./ pi. xxxvi, f. 9, 1S49. 

 B. Piizosiamis, D'Orbigny, ' Pal. Frany., Terr. Jur.,' j). 117, ])1. xvi, 

 f. 1800. 



This frequent species, or group of species, varies much in several important 

 characters. The degree of compression is by no means uniform, but I have seen no 

 example of alveofar compression a{)proaching to that rej)resented in D'Orbigny, fig. 4, 

 pi. xvi. All my specimens have in that part a slightly elliptical section. In some the sides 

 of the guard are flattened or a little grooved (PI. XXXI, fig. 77); the apicial region is some- 

 times unusually compressed, but generally follows the sweep of the sides ; the ventral groove 

 near the apex varies from little more than a mere flattening (PI. XXXII, fig, 79) to a 

 l)road furrow (fig. 76, v), a sharp short rift (fig. 77, v), a naiTow groove (fig. 76, y", 

 and fig. 78), and a deep lengthened canal (figs. 80 and 81). The general figure, always 

 long, varies in the proportion of length to diameter. 



Guard. Very long, subcylindrical, more or less compressed, tapering evenly to a 

 point, grooved on the ventral aspect from the apex through one third or more of the 

 length of the axis ; in perfect specimens this groove is often bistriated, or somewhat 

 sharply bordered. 



Sections show the axis placed nearer to the ventral surface ; in young specimens the 

 compression is considerable, growing less with age ; there is sometimes a distinct lateral 

 flattening on the middle part of the guard. 



Greatest length observed, in specimens from St. Neot's, 10 inches, of which the axis 

 is G inches; the diameter at the alveolar apex 1 inch. In another the diameter at the 

 alveolar apex is Ij inch. 



In a very young state the pearly lamina? about the alveolar apex are sometimes 

 decomposed, and the guard assumes the delusive shape of ' Actinocamax.' A drawing 

 has been shown to me in which this fusiform guard, or ' ossicle,' is represented as 

 separated from the ' nucleus ' of the phragmocone, but I have seen no specimen of the 

 kind. Prof. Owen figures ('Phil. Trans.,' 1844, pi. ii, fig. 4) a very young individual, 

 with the guard and alveolar chamber in their ordinary- relations. The guard is in this 

 state shorter in proportion than in after-life. 



Proportions. In a full-grown specimen from St. Neot's the diameter from back to 

 front, at the alveolar apex, being taken at 100, that from side to side is 90, the axis is 

 000; ventral radius 45, dorsal 55. The section is slightly oval, the ventral face rather 

 broader than the dorsal. 



In a young specimen, 3 inches long, the proportions of the diameters are also 

 100 to 90; the axis is more excentric than in the older specimen, the ventral radin- 

 being only 30, the dorsal 70, the axis 050. 



