534 



MISS G. L. ELLES OlST THE GEAPTOLITE [Aug. 1 898, 



species of Bryograptus has not yet been recognized. Subsequent 

 development seems to have proceeded along two quite different lines, 

 resulting in one case in Phyllograjptus angustifolius, Hall ?, and in 

 the other in Didymograptus gihherulus, Nich. 



Between Tetragraptus Bigshyi and Phyllograptus there appears 

 to be an intermediate form, but I have not seen it in England. 

 It has been recognized in Sweden, however, where it has received 

 the name of T. pliyllograptoides (Linnarsson, MS.). In this form 

 the four stipes are united in the proximal part of the rhabdosoma, 

 but are free at their distal ends ; where the four stipes are united 

 for their whole length the form is termed a Pliyllograptus. The 

 development of Didymograptus gihherulus is, I consider, more direct, 

 the main cause being the absence of dichotomous division in the 

 common canal. 



In this group the very close resemblance in the characters of the 

 proximal end in the various forms is worthy of notice. In all the 

 forms we note the marked similarity in size and general form of the 

 sicula and the earliest developed thecse, which differ in direction of 

 growth from all those subsequently formed. A comparison of the 

 structure of this part of the rhabdosoma as worked out by Holm 

 for T. Bigshyi, D. gihherulus, and Ph. angustifolius ^ will bring this 

 out clearly. Other resemblances between these species are shown in 

 the following table : — 



Species. 



Tetragraptus Bigshyi. 



Tetragraptus phyllograp 

 toides. 



Phyllograptus angusti- 

 folius ? 



Bidymograptus gihherulus 



Cell-characters, etc. 



^ 



Cells long and 



curved, in contact 



)■ for whole length, 



I and with con- 



1 cave apertures. 



No. of 



cells 



to inch. 



Incli- 

 nation. 



Aper- 



tural 



angle. 



Over- 

 lap. 



32-36 

 (13-14) 

 36 (14) 



40-50° 



140° 



whole 



32 (13) 



variable 



variable 



>j 



40 (16) 



45<^ 



130° 



)> 



(e) Group containing forms derived from Bryograptus Callavei. 



The forms which fall into this group include a Tetragraptus of 

 the HicJcsii-iype, but not certainly T. Hichsii (Hopk.) itself, Didymo- 

 graptus Nicholsoni, Lapw., D. affinis, Nich., D. gracilis, Tornq., 

 Azygograptus LapwortJii, Nich., and Az. suecicus, Moberg. 



Nicholson & Marr have already called attention to the relationship 

 between Bryograptus Callavei, Lapw., and Tetragraptus Hicksii. 

 The latter species has not yet been recognized in the Skiddaw Slates, 

 and probably the form from which the simpler types have been 

 derived may be somewhat different from that species, though closely 

 allied to it. Prom this type two forms of Didymograptus were, I 



' ' Om Bidymograptus, BTiyllograytus, och Tetragraptus^ Geol. Foren. Stockh. 

 Forhandl. vol. xvii (1895). 



