98 



BEITISTI TtRAPTOLITES. 



The best specimens are in Tiapwortli's collection and that of the Woodwardian 

 Mnsenm. 



Character-'^ of the different f^pecies of Azygograpti. 





Group I. 



Group II. 



Group III. 



Az. Lapivorthi 



Az. Hicksii. 



Az. suecicus. 



Az. coelehs. 



Character of stipe 



Flexed 



Flexed 



Kigid. 



Straight and 

 horizontal 



Maximum widtli of stipe . . . 



1 mm. 



1-6 mm. 



0-8 mm. 



0-5 mm. 



Length of sicula 



15 mm. 



1-7 mm. 



16 mm. 



0-9 mm. 



Origin of ill. 1^ 



Central 



Central 



Suboral 



Apical 



No. of thecal in 10 mm. . . . 



7-8 



4—5 



7-8 



10 



Inclination 



20'^ 



10°— 15° 



15° 



15° 



Overlap 







0-i 



0— i 



1. — X 



Average apertural angle . . . 



90° 



90°- 100° 



90° 



85° 



Genns PHYLLOGRAPTUS, Had. 



1858. Phyllograptus, Hall, Geol. Survey Canada Eep., 1857, p. 135. 



PoJi/jMry foliiform, bilaterally symmetrical, consisting of four uniserial main 

 stipes, Avliich are produced by dichotomous division from a primitive 

 Didynwgrapfns stage, and which coalesce by the whole of their dorsal 

 surfaces. Sicula embedded. 

 ThecdB single cylindrical or subcylindrical tubes of approximately uniform 

 width, usually in contact throughout their length. 

 The species included in this genus constitute a group which might almost cause 

 them to be regarded as merely a sccvndeiit series of Tetragrapti, were it not that a 

 new feature is introduced into their structure whereby the stij)es are actually fused 

 along the entire length of their dorsal walls, and constitute as it were two inter- 

 secting ovals. 



The initial stages of development appear to be in all respects comparable with 

 those described for Tetragraptus. 



