SUBCLASS II 



DIBRANCHIATA 



681 



convex toward the front ; closely resembling Orthoceras. Septa rather distantly 

 spaced ; sipliuncle marginal, thin ; proostraciim unknown. Guards of this genuH 

 are rare, biit detached phragmacones are not unconunon. Upper Alpine Trias. 



Atractitas Gümbel. Like Äulacoceras, but guard large, smooth and witliout lateral 

 furrows. Phragmacone either smooth, or with fine asymptotic lines, and dorsal area 



marked with extremely fine 

 growth - lines, convex toward 

 the front. Guards and phrag- 

 macones almost always occiir 

 detached. The latter were 

 originally mistaken for Ortho- 

 ceratites, but are distinguished 

 by their marginal siphuncle 

 Hü-iiivGiaMMiu -; xeAruniiim. ^^^b^xw^kh ^nd characteristic conothecal 



Striae. Upper Trias and Lias 

 of the Alps ; also Trias of 

 California. 



Fig. 1315. 



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

 above the phragmacone. li, Belemnites hrugierianuH Miller. Lower 

 Lias ; Charmouth, England. Impression of complete individnal. Vs 

 (after Huxiey). ö, Restoration of a Belemnite shell. 



Abbreviations : R, Rostrum or "guard"; l'h, Phragmacone; Po, 

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

 alveolus («) ; h, Impression of arms ; c, Camerae of phragmacone ; i, An- 

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



Fig. 1316. 



Belemniteft compressus. Lias ; 

 Gundershofen, Alsace. 

 Phragnuicoiie with well-pre- 

 served conotheca. «, Asymp- 

 totic line ; h, Hyperbolic 

 area ; v, Ventral area. 



Xiphoteuthis Huxiey. Middle Lias ; England. X elongata Huxiey. 



Belemnites Lister (Figs. 1315-1319). Name first applied by Agricola in 1546. 

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

 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 diflerent ages 

 frequently dilfer considerably in form. Such differences are well illustrated in />. 

 acuarius Schloth. The young are sometimes fusiform, l)ut grow cylindiical or 

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



