BELEMMTES OF THE LIAS. 



77 



of the phragmocone withiu the slightly oval guard, which is thickened on the dorsal 

 aspect ; those across the post-alveolar region show the axis to be very excentric till near 

 the apex (PI. XVI, figs. 5-" s"). 



Longitudinal sections give a nearly straight axis, prolonged in the direction of the 

 ventral line of the phragmocone. 



Proportions. Taking the dorso-ventral diameter at the alveolar apex at 100, the 

 ventral part is less than 40, the dorsal more than 60, the cross diameter 95, and the axis 

 180 to 200 (in young specimens 300). Between the apex of the phragmocone and the 

 apex of the guard the dorsal radius is sometimes twice as long as the ventral. 



Phragmocone. Somewhat oval in section, slightly arched toward the ventral side ; 

 angle 28°. 



Locality. Very abundant in ironstone-beds belonging to Middle Lias, between the 

 jet-rock and marlstone, east of Staithes, Yorkshire {Phillips). 



On a group or Belemnites allied to belemnites compressus of 

 Blaixville and Voltz. 



The title "Belemnites compressus" appears to have been first used by Stahl, as already 

 noticed, p. 41. But the papers of this author in the ' Correspondenzblatt der Wiirt. 

 Landw. Vereins,' 1824, attracted little notice, and the greater number of Belemnitologists 

 followed the example of Blainville, 1S27, who gave the name of "compressus" to a different 

 species, belonging to a different group. In his description the essential points are as 

 follows : 



Shell straight, very compressed, so that the vertical diameter is much greater than the 

 transverse, and the section is oval ; apex medial, straight, with a broad shallow groove on 

 each side, dying out by degrees toward the alveolar region ; alveolar cavity conical, with 

 an oval section, six inches and more long. PL ii, figs. 9 and 9 a, of Blain\ille's work 

 represent the Belemnite laterally and in cross section. From these figures I infer that 

 the axis of the guard was above three times as long as the diameter at the apex of the 

 phragmocone. Such a specimen may be found in the sandy beds at the base of the 

 Lower Oolite of Yorkshire, while in a higher calcareous rock occurs another and allied 

 form, with five furrows at the apex, called by Blainville B. quinqiiesulcatus, and repre- 

 sented in his work, pi. ii, figs. 8, 8 a, 8 b. 



Voltz, writing in 1850, gives a general character of B. compressus, and places under 

 this title three varieties. The general characters are these : 



Sheath large, straight, conoidal or conical, compressed, with an oval cross section in 

 the alveolar region. Summit straight, emousse, furnished with two dorso-lateral furrows, 

 which pass over at least one third of the apicial region. Axis of the guard excentric, 



11 



