

i: KI'TILIAN AGE. J U RAS8IC PERIOD. 125 



iu, MOJCTF. 1808, ib. 390. 

 .nit,,, PLKHI.XO, Brit. Aniiu. 1828, 240. 

 Nutotifihiirs, lifVAL-Jucm, Beleniu. 1841, 64. 

 (ititlruti/ihiln, lU'v AL JOCVK, ib. 

 Ktym. Rixtion, a dart. 

 rM/<. HrlrlHiiitn J'uidlidHUt, D'OBBIOKY. 



IVn ci)iisi>tin^ of two parallel, nacreous, sword-snapcd processes, extending for- 

 ward 1'roin the anterior dorso-lateral margins of the phragmoconc. Guard elongated, 

 cylindrical, more or less clavate, or somewhat compressed; becoming very thin 

 ant< TiorK. when- it i-^ pierced by a'decp conical cavity with entire margins, for the 

 reception of the phragtnocone ; solid and more or less pointed at the posterior 

 e\tn mit\. 1'hragmoconc often terminating in a minute bulb at the apex ; septa 

 nearly hori/ontal, concave ; siphon contracted where it passes through the septa, 

 and somewhat expanded between them. 



Animal unknown. 



The guards of Belemnitcs, popularly called "thunderbolts," are the part most 

 frequently met with. They generally have a semi-translucent, somewhat horny 

 appearance, and a fibrous structure, the fibres radiating from a longitudinal, sub- 

 central lino. It varies greatly in form and size in the different species and varieties, 

 as well as, apparently, in the two sexes. Sometimes it only extends about half an 

 inch beyond the phragmocone, while in other instances it attains a length of one 

 or two feet. It is readily distinguished from the guard of the allied genus Bdem- 

 n it'll, i liy the absence of a slit down the anterior wall of the pierced end, and by 

 having no distinct vascular markings on the ventral side. It also wants the flattened 

 ridge always seen on the dorsal side of well preserved specimens of J.'clentnifil(<i. 



The genus has been divided into the following sections and subsections :' 



BKTIOH 1. Acoeli, BRUSH. Without dorsal or ventral grooves. 



a. Aeunrii, without lateral farrows, bat often channelled at the extremity. (Lias and Neocomian). 

 Type. B. aeuariai. 



b. Clarati, with lateral furrows. 

 Type. B. clnnilus (Lias). 



SECTION 2. Ciaatrocoeli, D'OBBIOKT. = \otOHiphites, DCVAL. Ventral groove distinct. 



a. Canaliculati, no lateral farrows. (Oolites.) 

 Type. Ii. ranaliculntui. 



b. llastali, lateral farrows distinct. (Upper Lias and Gault.) 

 Type.B. kaitatut. 



Snmoif 3. .\otocoeli, D'ORBIOXT. = Gastroaiphites, DI-VAL. With a dorsal groove and farrow on each 



side. 

 Type.B. dilatatut (Neocomian). 



The great numbers of the guards of Bclcmnites often found imbedded together in 

 the same stratum indicate that these mollusks were gregarious in their habits, and 

 they are supposed to have preferred muddy bottoms. The genus made its appear- 

 ance near the beginning of the Liassic epoch, during which it seems to have attained 

 its greatest development. It continued, however, to exist until about the middle 

 of the Cretaceous period. 



1 Woodward's Man. Mol. p. 74 



