484 Dr. Gerhard Holm — On the Structure of Graptolites. 



not seem to be fn any way impossible. It is uncertain whetber the 

 Graptolite iu question is only a young form or a distinct and fully 

 developed species, the development of which ceased at this stage. 

 How it comes about, I have not yet been able to decide. I have not 

 found with it any species to which it could be referred. If a right 

 branch should really be developed, the form might be referred to the 

 genus Azygograptus, Nich. and Lapw., in which only one branch 

 grows out from the free sicula. How and from what side of the 

 sicula the branch arises in Azygograptus, does not definitely appear 

 from the descriptions and figures given of the forms placed under it. 

 Probably the present form is a transitional type between Didymo- 

 graptus and Azygograptus. 



Genus Tetkagraptus, Salter. 



HalP first described the proximal end in Tetragraptus : "In the 

 Graptolites with four stipes the condition appears like that of two 

 individuals of two-stiped forms, conjoined by a straight connecting 

 process of greater or less extent, with the radicle point in the centre, 

 though often obscurely marked. This connecting process is always 

 destitute of cellules, and this with its divisions I have termed the 

 funicle." Upon this statement is founded the presence of a tubular 

 funicle uniting two sides of the polypary in forms with four or more 

 stipes belonging to the Dichograptida3. Almost all later authors, 

 as, for example, Lapworth and Nicholson, have followed Hall in 

 believing that the funicle is destitute of thecse. I, however, have 

 described the presence of a theca on each side of the sicula in the 

 funicle in Trochograptus diffusus, Holm, whose proximal end is 

 similar to that of Tetragraptus. 



On these grounds, and in consideration of the many branched 

 Dichograptidse being embedded in shale, and therefore showing the 

 thecse of the central part of the polypary only in very exceptionally 

 favourable cases, and as these thecee are analogous to those in 

 Didymograptus and other forms which are better exposed, I draw 

 the conclusion that the funicle in many cases, if not always, was 

 furnished with thecge. 



Curiously enough, Tornquist believes (Siljansomradets Grapt., i, 

 p. 15) that "the expanded end of the sicula in Dichograptidse which 

 have more than two primary branches divides repeatedly into two, 

 and thus without apparent border passes over into the theca-bearing 

 polypary " ; and it appears that on account of the supposed difference 

 between Didymograptus and the remaining Dichograptidge, he is in- 

 clined to separate the former as the type of a distinct family — 

 Uidymograptidse. 



A " funicle " has not been found in any Graptolite. 



TetraCxRaptus Bigsbyi, Hall. PI. XIII, Figs. 9-16; PI. XIV, 

 Figs. 13-16 ; Figs. 1-6, p. 485. 



This is the only form of Tetragraptus upon which I can offer 

 any remarks. 



^ "Graptolites of the Quebec Group " : Geol. Suit. Canada, dec. ii, 1865, p. 19. 



