Roijal Society. 1 27 



isting species. When the grinders are below the typical number, 

 the missing molars are taken from the back part of their series, and 

 the premolars from the fore part of theirs : the most constant teeth 

 being the fourth premolar and first true molar ; these are always 

 determinable, whatever be their form, by the relation to them of the 

 last tooth of the deciduous series. Thus determined, the homo- 

 logies of the other grinders are ascertained by counting the molars 

 from the first backwards, 1, 2, 3 ; and the premolars from the last 

 forwards, 4, 3, 2, 1. The symbols are made by adding the initial m 

 to the numbeis of the molar teeth, and the initial jo to those of the 

 premolar teeth. The author concludes by pointing out the advan- 

 tages of this system of anatomical notation. 



Feb. 14. — 1. "Supplementary Observations on the Structure of 

 the Belemnite and Belemnoteuthis." By Gideon Algernon Mantell, 

 Esq., LL.D., F.R.S., Vice-President of the Geological Society, &c. 



In this communication the author describes his recent investiga- 

 tions on the structure of the two genera of fossil Cephalopoda, whose 

 remains occur so abundantly in the Oxford clay of Wiltshire, 

 namely, the Belemnite and Belemnoteuthis, as supplementary to his 

 memoir on the same subject, published in the Phil. Trans. 1848*. In 

 that paper evidence was adduced to show the correctness of the 

 opinion of the late Mr. Channing Pierce as to the generic distinction 

 of these two extinct forms of Cephalopoda. 



As however several eminent naturalists had expressed doubts as 

 to some of the opinions advanced by the author in his former memoir, 

 figures and descriptions are given in the present notice, of beautiful 

 and instructive specimens lately discovered in Wiltshire, and which 

 he conceives establish his previous conclusions. Dr. Mantell then 

 states as the result of his examination of several hundred examples, 

 that our knowledge of the organization of the animal of the Belem- 

 nite is at present limited to the following parts, viz. — 



1. An external Capsule or periostraciim which invested the osse- 

 let or sepiostaire, and extending upwards, constituted the external 

 sheath of the receptacle. 



2. The Osselet, characterized by its fibrous radiated structure, 

 terminating distally in a solid rostrum or guard, having an alveolus, 

 or conical hollow, to receive the apical portion of the chambered 

 phragmocone ; and expanding proximally into a thin cup, which 

 became confluent with the capsule, and formed the receptacle for 

 the viscera. 



3. The Phragmocone, or chambered, siphunculated, internal shell ; 

 the apex of which occupied the alveolus of the guard, and the upper 

 part constituted a capacious chamber, from the basilar margin of 

 which proceeded two long, flat, testaceous processes. 



These structures comprise all that are at present known of the 

 animal to which the fossil commonly called " The Belemnite" 

 belonged. 



Of the Belemnoteuthis, the fossil cephalopod which Prof. Owen 



* An abstract of which appeared at p. .388 of the first volume of the pre- 

 sent series of this Jomnal. 



