504 



BRITISH GRAPTOLITES. 



Specific Characters of 



Forms belonging 



to the Genus 



MONOGRAPTUS. 







GROirP VII.— (RASTRITES). 



M. pere- 

 grinut. 



M, lortgi- 

 tpinut. 



31. »etiger. 



M. hyhridus. 



M. approxi- 

 matus. 



M. 

 ftigax. 



M. Ltnncpi. 



maxtmus. 



M. egui- 

 dittans. 













var. 

 Geinitzi. 





1 





Character of polypary 



Fish- 

 hook 

 form 



Broadly 

 arcuate 



Proxi- 



mally 



almost 



straight, 



irregular 



distally 



Small, 

 arcuate, 



broadly 

 recurved 

 proximal 



portion 



Convolute 



pi'oximal 



portion, 



arcuate 



distally 



Arcuate 



Arcuate, Slightly 

 with arcuate, 

 irregular thecae 



distal on 

 curvature convex 

 side 



Irregu- 

 larly 

 flexed 



Characters of theciE : 

 1. No. in 10 mm. 



8-10 



7—8 



12 



8—10 



10-12 



6—8 



6—4 3—2 



4-2 



2. Length 



25 mm. 



5 mm. 



5 mm. 



2 mm. 



4 mm. 



1 mm. 



8 mm. IS cm. 



5 mm. 



3. Proportion of 

 length to width 



2i 



5 



6 



2 



4 



^ 



+ 3 



2-2i 



Equal 

 1:1 



of interspace 

 (mature) 





















Genus CYRTOGRAPTUS, Garruthers. 



1867. Cyrtograpsus, Carruthers. Murchison's Siluria, 4th. Edition. Fossils 90, fig. 1, p. 541. 



Polypary unilateral, compound, with more or less spiral mode of growth. 



T/iecce conical or tubular; of the type of those of Monograptas Nilssoni with 

 apertural margins straight, or of the type of those of M. spiralis with 

 apertural margins reflexed. 



In the various forms of Monograptidae, known collectively as Gyrtograpti, the 

 mode of origin of the so-called " branches " is peculiar. As a general rule the 

 appearance presented is that each so-called " branch " originates either directly or 

 laterally from the apertural region, or aperture, of certain thecse. 



Each " branch " is usually initiated by a small sac-like or vesicle-like structure, 

 through which the " branch " eventually appears to break its way (see figs. 

 351 a — (•) ; and the presence of this initial " sac " gives an appearance of unusual 

 length to the first "theca" of each "branch." This apparent mode of branching 

 is very unlike that described by us as " dichotomous " in the cases of the families 

 of the Dichograptid^, Leptograptidce, etc. 



While these " branches " in Cyrtograptiis originate in some species at fairly 

 regular intervals along the course of the stipe, there is no constant rule, even in 

 examples of the same species, and in some forms only a single "branch" is shown. 

 Indeed, the appearances presented are suggestive of the view that the so-called 



