108 Belemnitella Americana and Mucronata [306 



from a trustworthy guide to the adaptable characteristics of 

 the form, vitiates to a certain extent any general conclusions, 

 such as that wide-spread occurrences of type fossils indicate 

 correspondingly widespread similarity of life conditions. 

 Nevertheless, the apparent inability of these Belemnitellas to 

 live except in a certain, rather fixed environment, is at least 

 suggestive that their powers of adaptation were limited. 



Whitfield (1) describing the American species writes as 

 follows : 



" Stylet or guard, rather large, solid, and heavy, often becoming 

 thickened with age. Specimens varying from nearly 3 inches to 

 nearly 4 inches in length below the base of the slit, the larger ones 

 evidently having a length of fully 6 inches, from the lower extremity 

 to the top of the internal cavity, or conotheca. General form triangu- 

 larly cylindrical in the upper part, becoming flattened on the ventral 

 side in the lower part, with frequently a slight mucronate extremity. 

 . . . The upper end of the stylet or guard, from about the base of the 

 internal cavity, gradually expands upward, and becomes very thin on 

 the edge, and the inner surface of the wall often bears the marks of 

 the transverse septa of the phragmacone. The entire surface is 

 usually much roughened When not worn, the roughening being great- 

 est on the ventral side, while laterally this roughening produces vas- 

 cular lines running obliquely backward, in crossing from the ventral 

 to the dorsal surfaces, and on the raised lanceolate, area of the 

 dorsal surface the markings are finer and arranged so as to produce 

 longitudinal lines, or interrupted striae." 



There seems to be a fair degree of unanimity as to the 

 similarity of B. americana (Morton) and the European form, 

 B. mucronata, D'Orbigny. Morton says (2, p. 190). 



" This species has an analogue in the B. mucronata of Schlotheim, 

 which is characteristic of the Chalk throughout Europe. It seems 

 also to resemble the belemnite of Maestricht, as figured by Faujas." 



D'Orbigny, (3, p. 63-4) describes the European form as 

 follows: 



"Rostre" allonge", quelquefois un peu comprime", cylindrique sur sa 

 moitie" ante"rieure, de 1& acumine" jusqu'a 1'extremite" tres -obtuse, au 

 milieu de laquelle est une pointe souvent assez allonge"e; les deux 

 impressions dorsales sont tres marquees, large, et il en part des 

 petits sillons ramifies et reticule's, qui viennent joindre le partie in- 



