596 



MOLLUSCA 



SUB-KINGDOM VI 



phragmocones almost always occur detached. The former originally mistaken for 

 Orthoceratites, but are distinguished by their marginal siphuncle and characteristic 

 conothecal striae. [Hyatt regards both Aulacoceras and Atractites as descended from 

 Orthoceratites.] Upper Trias and Lias of the Alps. 



Xiphoteuthis, Huxley. Lower Lias ; England. 



Belemnites, Lister (Figs. 1237-1242). Name first applied by Agricola in 1546. 

 Guard dactyliform, subcylindrical, or conoidal, sometimes short and thick, sometimes 



A, Vertical section of a Belemnite, the proostracum broken away 

 above the phragmocone. B, Bclemnites Brugierianus, Miller. Lower 

 Lias ; Channouth, England. Cast of complete individual. 1/3 (after 

 Huxley). C, Restoration of a Belemnite shell. 



Abbreviations: R, Rostrum or "guard"; Ph, Phragmocone; Po, 

 Proostracum ; a, Apical line reaching from apex of guard to bottom of 

 alveolus (o) ; &, Impression of arms ; c, Camerae of phragmocone ; i, An- 

 terior end of proostracum ; o, Protoconch ; si, Siphuncle ; x, Ink-bag. 



FIG. 1238. 



Aulacoceras reticulatum, Hauer. 

 Upper Trias ; Rothelstein, near 

 Aussee, Austria. A, Guard and 

 phragmocone, 2 / 3 . B, Guard, Vi- 

 C, Portion of phragmocone sliced 

 to show siphuncle and siphonal 

 funnels. 



slender and much elongated ; retral portion tapering, submucronate, or obtusely 

 rounded. Owing to irregularity in secretion of calcite layers on the periphery of the 

 guard during growth, individuals belonging to the same species but of different ages 

 are apt to vary considerably in form. Such differences are well illustrated in B. 

 acuariuSj Schloth. The young are sometimes fusiform, but grow cylindrical or 

 conical with age. About 350 species are known, ranging from the Lower Lias to 

 summit of the Cretaceous; maximum from Middle Lias to Lower Cretaceous. 



