117 



ON THE PROOSTRACUM OF A BELEMNITE FROM THE UPPER 

 LIAS OF ALDERTON, GLOUCESTERSHIRE. 



By G. C. CiiiCK, F.G.S., etc. 



Mead 8th May, 189G. 



PLATE IX. 



The shell of the extinct genus Belemnites consists fundamentally 

 of— 



1. A hollow cone, the phragmocone (with a thin shelly wall, termed 

 the conotheca), divided by transverse septa, concave above and convex 

 below, into chambers or loculi : the septa are perforated near the 

 ventral margin by a siphuncle. 



2. A guard or rostrum, more or less extensively enveloping the 

 apical part of the phragmocone. 



3. A dorsal plate or anterior shell, which is a very thin plate 

 extending forwards on the dorsal side, and considerably beyond the 

 chambered portion of the phi'agmocone. To this Professor Huxley, 

 in his famous memoir on the structure of the Eelemnitidaj, gave the 

 name proostracum. 



The guard or rostrum is the portion of the shell which is usually 

 preserved ; frequently the phragmocone also is found ; but the remains 

 of the proostracum are so seldom met with in anything like a perfect 

 state of preservation, that it seems desirable to place on record 

 a specimen which has lately been acquired by the British Museiim. 

 It formed part of the Rev. P. B. Brodie's collection, and is labelled, 

 in (as I am informed) Mr. Brodie's handwriting — " Sepiaceous portion 

 of perhaps a belemnite. Upper Lias : Alderton, Glostershire. /77." 

 The words " a belemnite " have been erased in pencil, and the 

 words " Like Beloteuthis''^ have been added in ink. 



The specimen is displayed on the two counterpart faces of a split 

 slab of limestone, here designated A and B respectively. 



It is not possible to determine the species of Belemnites to which 

 the proostracum belongs, since nearly the whole of the guard is 

 missing ; but that it belonged to a Belemnite is shown by some 

 fragments of the ■ anterior part of the guard which are associated 

 with the remains of the phragmocone, and exhibit the characteristic 

 structure of this portion of the shell. 



The phragmocone is so much crushed that its exact form cannot be 

 ascertained. Its fragments are associated with a dark-bro"\vn, almost 

 black substance, evidently the fossilized contents of the ink-bag. On 

 the one slab (A) this brown stain does not extend beyond the anterior 

 part of the phragmocone, but on the other (B) the stain reaches 

 quite to the anterior border of the proostracum. Since this stain is 



