THE FAMILY TEXTULAEIID^ 167 



The triserial arrangement of the segments is very 

 clearly seen in this species. The angles of the 

 segments forming the salient edges of the test 

 are prolonged by exogenous shell growth into spines, 

 which in specimens from clean and well-preserved 

 dredgings are sometimes of considerable length and 

 delicac}'. Besides being found in the Chalk this 

 species is frequent in many succeeding formations, as, 

 for instance, the Miocene of Austria, Bavaria, and 

 Malta, the Pliocene of Italy, and others of still later 

 age. By its occurrence in recent dredgings we find 

 it is most at home in depths of less than 100 fathoms. 

 (Plate 9, fig. D.) 



Genus Tritaxia, Eeuss. 



Test triserial, sometimes dimorphous, with a 

 simple produced central aperture. Lower Cretaceous 

 to Becent. 



Example. — T. tricarinata, Eeuss, ' Verstein. 

 bohm. Kreideform.' pt. i. 1845, p. 39, pi. viii. 

 fig. 60. 



This typically Cretaceous form is still represented 

 very sparingly in our present seas. It was an 

 abundant form in Upper Cretaceous times, and is 

 most characteristic of the Lower Greensand, the 

 Upper Gault, and the Chalk-Marl in England. 

 Lower Cretaceous to lleceiit. (Plate 9, fig. E.) 



Genus Chnjsalidiua, D'Oebigny. 



Test triserial (sometimes dimorphous — that is to 

 sa}', triserial at commencement and afterwards 



