SUBCLASS II 



DIBRANCHIATA 



681 



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

 spaced ; siphuncle marginal, thin ; proostracum unknown. Guards of this genus 

 are rare, but detached jshragmacones are not uncommon. Uj^per Alpine Trias. 



Atractites Giimbel. Like Aulacoceras, but guard large, smooth and without lateral 



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



A B c marked with extremely fine 



growth - lines, convex toward 

 the front. Guards and phrag- 

 macones almost always occur 

 detached. Tlie latter were 

 originally mistaken for Ortho- 

 ceratites, but are distinguished 

 by their marginal siphuncle 

 and characteristic conothecal 

 striae. Upper Trias and Lias 

 of the Alps ; also Trias of 

 California. 



Ph 



Pig. 1315. 



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

 above the phragmacone. B, Behmnltes hrugicrlanus Miller. Lower 

 Lias ; Cliarmouth, England. Impression of complete individual. 1/3 

 (after Huxley). C, Restoration of a Belemnite shell. 



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

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

 alveolus (0) ; h, Impression of arms ; c, Camerae of phragmacone ; i, An- 

 terior end of proostracum ; 0, Protoconch ; si, Siphuncle ; x. Ink-bag. 



Pio. 1316. 



Bclcmnitescomprcssiis. Lias ; 

 Gundershofen, Alsace. 

 Phragmacone with well-pre- 

 served conotheca. a, Asymp- 

 totic line ; h. Hyperbolic 

 area ; v, Ventral area. 



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



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

 Guard dactyliform, subcylindrical or conoidal, sometimes short and thick, 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 .sjjecies but of difierent ages 

 frequently differ considerably in form. Such difterences are well illustrated in B. 

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

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



