RETIOGRAPTUS. 



315 



Table of Specific Characters of the Genus Glossogeaftus. 





GLOSSOGRAPTUS. 



G. cfr. ciliatus. 



G. Hincksii. 





G, armatus. 



G. acanthus. 







var. fimbriatut. 







Character of polypary . 



Broad, parallel- 



Fairly long, 



Smaller than 



Small 



Subfusiform, 





sided 



widening 

 rapidly 



typical form 





widening 

 cpaickly 



Maximum width .... 



4 mm. 



3 mm. 



2 mm. 



2 mm., uni- 

 form 



4 mm. 



Character of proximal end 



Broad 



Narrow 



— 



With long 

 flexed spines 



— 



Characters of theca; — 













(1) No. in 10 mm. 



10 



16—10 



— 



9 



10 



(2) Apertural margin . 



Horizontal 



Horizontal or 



slightly everted 



— 



Slightly 

 everted 



Everted 



(3) Ornamentation — 













(a) Apertural spines 



Long 

 and delicate 



Strong arcuate 



— 



Slender, rigid 



Robust, rigid, 

 slightly curved 



(b) Septal spines 



? 



Straight, spur- 

 like 





Straight, stiff 



? Eobust, stiff 



Genus RETIOGRAPTUS, Hall. 



1865. Retiograptus, Hall, Grapt. of Quebec Group, Geol. Survey Canada, dec. 2, p. 115. 

 1873. Clathrograjptus, Lapworth, Geol. Mag., vol. x, p. 559. 



Polypary bilaterally symmetrical, truncato-elliptical or sub-hexagonal in trans- 

 verse section ; septal, apertural and parietal lists usually present, and 

 tending to form a more or less complete supporting skeletal framework or 

 clathria. Ventral and septal processes unknown. 



Thecse of the general Orthograptus type. 



Test membranous, continuous, greatly attenuated. 



Only two species of this genus are known in Britain. The first was originally 

 described by Lapworth as Clathro grapt us cuneiformis. The researches of Ruedemann, 

 however, make it evident that this form is certainly congeneric (if not identical 

 with) the original Retiograptus Geinitzianus of Hall, which name has the priority, 

 though it is a singularly unfortunate one. The second is the form described 

 by Lapworth as Idiograptus aculeatus, which appears to be a very close ally of 

 Hall's Retiograptus eucharis. 



The affinities of Retiograptus are somewhat doubtful ; it is here regarded as 

 being more or less intermediate between Glossograptus and Retiolites. It agrees 

 with Glossograptus in the general continuity of its test and the shape of the thecse ; 

 but the clathria is much more strongly differentiated as such. It approaches 



