PHRAGMOPHORA. 877 



diverge slightly and become shallower as they extend forwards, and 

 which mark the dorsal side of the shell. 



Between three and four hundred species of the genus Belemnites 

 are known, the maximum development of the group taking place in 

 the Jurassic rocks. The oldest forms appear in the Lower Lias, and 

 the last in the Chalk, but the genus appears to have wholly died out 

 with the close of the Cretaceous period. 



According to Zittel, the following sections of the genus Belenmites 

 may be recognised : — 



1. Acuarii. — Guard conical, with two or three apical grooves, but 

 without a " ventral furrow" or " dorso-lateral grooves." {Ex. B. acuarius, 

 Upper Lias.) 



2. Canaliculati. — Guard elongated, conical, or fusiform, with a deep 

 " ventral furrow." {Ex. B. canaliculars, Inferior Oolite, fig. 806, C.) 



3. Clavaii. — Guard elongated, more or less clavate posteriorly, without 

 a "ventral furrow," but with well-marked lateral grooves. {Ex. B. 

 davatus, Lias.) 



4. Bipartiti. — Guard slender, cylindrical, with or without a " ventral 

 furrow," but with deep " dorso-lateral grooves." {Ex. B. bipartiius, 

 Neocomian.) 



5. Hastati. — Guard elongated, narrower in front, thicker behind, and 

 terminating in a point posteriorly. A deep " ventral furrow " is present, 

 together with shallow lateral grooves. {Ex.B. liastacus, Oxford Clay.) 



6. Conophori. — Guard conical, pointed behind. • No ventral furrow is 

 present, but a corresponding furrow proceeds from the edge of the 

 alveolus backwards on the dorsal aspect of the shell. 

 Lateral grooves are wanting or feebly developed. {Ex. 

 B. conophorus, Neocomian.) 



7. Dilatati. — Guard short, laterally compressed, flat- 

 tened or four-sided. A dorsal furrow, as in the preced- 

 ing group, is present, and the lateral grooves are more 

 or less developed. {Ex. B. dilatatus, Neocomian.) 



Closely allied to Belemnites proper, and probably 

 not generically separable, are the forms included 

 under the name of Belemnitella. In these types 

 (fig. 808) the guard is cylindrical, with a short 

 pointed mucro behind. Two diverging dorso-lateral 

 lines exist on each side ; and there is a slit-like 

 ventral furrow, which begins at the margin of the 

 alveolus, but does not reach the hinder end of Fi 8o8 _ Belem . 

 the same. The ventral side of the guard shows mteiiamucronata. 

 very well-marked vascular impressions. The known 

 species of Bele7?mitella are confined to the Upper Cretaceous rocks. 



The most ancient type of the Belemnitidce. is the genus Aulaco- 

 ceras, which is found in the Upper Trias of the Alps. In this 

 genus the guard is elongated and clavate in form, with a deep 

 lateral groove on each side. The phragmacone is at least twice 

 as long as the guard, and slowly increases in width anteriorly. 



