CLASSIFICATION. 31 



diameter from side to side is less than that from the back to the front ; depressed when 

 the contrary occurs. The axis of the guard (called the " apicial line " by Voltz) is the 

 line from the apex of the guard to the apex of the phragmocone ; the axis of the phragmo- 

 cone extends from its apex to the centre of the last septum. The relative length of these 

 axes is of much interest and importance in diagnosis. Phragmocones are not always truly 

 conical ; they are usually a little compressed, and deserve to be called conoidal. The angle 

 of inclination of their sides is believed to be nearly constant in the same species, for the 

 same part of the slopes ; but it varies in different species (12° to 32°), and in the same 

 species it often varies a little between the apex and the last chamber. The angle most 

 proper to take for characters of species is between side and side ; but it is desirable, when 

 good examples occur, to give also the angle of the back and front. The section of the 

 alveolar chamber may often be had when the phragmocone cannot be obtained. 



BELEMNITIC BEDS. 



A full account of the geographical and geological distribution of Belemnites must 

 be postponed till the species have been described ; but it will be convenient here to 

 indicate the principal zones, or beds of rock, in which they are found most abundantly. 

 Not only is the group absent from existing oceans, but it is unknown in the whole 

 Csenozoic period; for Beloptera and Belemnosis, which occur in Eocene strata, are 

 probably of the family Sepiadse. 



In the Upper Chalk of Kent, Norfolk, and Yorkshire, with Ananchytes ovata, we 

 recognise in abundance Belemnitellse, the latest members of the family in Britain ; at 

 Maestricht beds of Chalk, thooght to be somewhat higher in the series, also contain them 

 in plenty. Comparatively rare in the lower parts of the Chalk, and not very frequent 

 in Upper Greensand, they are plentiful in the Gault, but again rare in Lower Green- 

 sand. No member of the family has been found in the Wealden strata. 



Belemnites appear below, but, excepting one dubious notice, I have no information of 

 them in any part of the Purbeck or the Portland strata. They are abundant in 

 Kimmeridge Clay, frequent in Coralline Oolite, less frequent in Calcareous Grit, but 

 again become plentiful in the Oxford Clay, both in the upper part and lower part, 

 as well as in the. Kelloway Rock. 



Again, they become rare in Cornbrash, and are almost unknown, except as fragments, 

 in Forest-Marble. In Bradford Clay they are unknown to me, except by a notice in 

 Smith's ' Stratigraphical System,' where a small slender species is quoted from Stoford, 

 south of Bradford, in Wiltshire. Nor have I more than mere fragmentary indications in 

 the Great Oolite, ^ill at the base of it we find the canaliculated Belemnites of Stonesfield. 

 Smith mentions a canaliculated species in the Fuller's Earth, but they are very rare in 

 any of the beds between the two Oolites of Bath. 



