560 



NEW YORK STATE MUSEUM 



the originally equal dimensions of their thecae, the differences found in the 

 closeness of arrangement and inclination of the thecae can, also, here be 

 attributed ^vith propriety to a condensation of the branches.^ 



(7) A small series of Tetragrapti and Didymograpti of the Deep kill 

 fauna is characterized by projecting acute or mucronate apertures of the 

 thecae. The accompanying table gives the detailed characters of the thecae 

 of these. 





Character of thecae 



Number of the- 

 cae in 10 mm 



Angle of diver- 

 gence 



Overlap 



B r y g 1- a p t u s 1 a p - 

 w r t h i 



Thecae slender 

 straight, with 

 abruptly widen- 

 ing projecting 

 apertures 



10 



15°-20° 



i-i 



Teti-agraptus tar- 

 axacum 



12 



4° proxim- 

 ally ; 40°- 

 60° dis- 

 tally 



^ proximally ; 

 J distally 



Di dymograptus 

 acutidens 



11 



15° 



i-i 



D i d y m g r a p t u s 

 e n s p i d a t n s 



11-12 



18° proxi- 

 mally;40° 

 distally 



i-i 



The table shows readily that the thecae of these species agree well 

 in their arrangement, inclination and overlap. The last three named 

 species are still more closely united by the character of the 

 aperture, mentioned above, which attains its extreme development in 

 Didymograptus cuspidatus, which also is the last appear- 

 ing form of the series. Bryograptus lapworthi does not possess 

 the typical apertural projections of the group. The fact, however, 

 that in Tetragraptus taraxacum the first thecae also fail to present 

 this character, but agree fully with those of B . lapworthi, is good 



^ The interesting problem of the tendency to a condensation of the thecae is treated 

 more fully in another chapter [p.569]. 



