186 PALEONTOLOGY 



species, and little notice has been taken of ontogeny, 

 although the mode of growth of the guard (by accretion) 

 makes changes in shape and grooving recognizable. 



The following is an imperfect statement of the classi- 

 fication most generally accepted. An asterisk denotes 

 genera always accepted as distinct, the remainder being 

 usually lumped in the comprehensive genus Belemnites. 



A. GUARD NOT GROOVED. 



Atrocities* Trias, Scythic to Rhaetic (guard nearly 

 cylindrical, alveolus nearly as long as guard, Fig. 53, c). 



Oxytenthis, Sinemurian-Charmouthian (guard acutely 

 conical, alveolus half length of guard), e.g. Bel. acutus 

 ( Fi g- 53> g)- 



B. GUARD WITH VENTRAL GROOVE. 



C&loteutkis, Sinemurian (short, cylindro-conical, alve- 

 olus nearly as long as guard, groove very wide), e.g. Bel. 

 excavatus (Fig. 53, h). 



Belemnopsis, Charmouthian-Bathonian (long, cylin- 

 drical, with acute apex, groove long, deep and narrow, 

 alveolus one-third length of guard), e.g. Bel. bessinus 

 (Fig. 53, d). 



Pachyteuthis, Charmouthian-Neocomian (stout, cylin- 

 drical, with conical apex, groove long and shallow, 

 alveolus from one-third to two-thirds length of guard), 

 e.g. Bel. abbreviates (Fig. 54, a). 



Cylindroteuthis, Callovian-Neocomian (long, cylindrical, 

 groove deepest near apex, becoming indistinct forwards, 

 alveolus about one-fourth length of guard), e.g. Bel. oweni 

 (Fig. 54, ft). 



Hibolites, Aalenian-Cenomanian (hastate), e.g. Bel. 

 hastatus (Fig. 53, e). 



Belemnitella,* Emscherian-Maestrichtian (cylindrical, 

 with rounded and acuminate apex, alveolus one-third 

 to one-half length of guard, ventral slit in guard along 



