438 Br. Gerhard Holm — On the Structure of GraptoUtes. 



and genei'a first enumerated, results from the mode of origin of the 

 same, as a prolongation of the initial part of the sicula. 



Tornquist (Siljansomradets Grapt., i, p. 16) has already pointed 

 out, in the descriptions of Didymograptus minutus, Tornq., that the 

 thread-like prolongation of the sicula most nearly corresponds with 

 the vii'gula in the genera Biplograptus and Monograptus. That 

 this must be the case is evident from Wiman's description of the 

 origin of the virgula from the sicula in Diplograj^tus. Whilst it 

 has been previously supposed that the virgula extended lengthways 

 along one side of the sicula, and therefore in the developed polypary 

 often formed a free, thread-like prolongation of the proximal as well 

 as the distal end of the same, Wiman has shown that the virgula 

 of the sicula, in the sense which he, in conformity with former 

 authors, gives to it, is not continuous, but consists of two separate 

 parts. The one of these, which forms the beginning of the true 

 virgula, stretching throughout the whole of the pol3^pary, originates 

 fiist near the apex of the sicula as a prolongation of the same, and 

 in Diplograptus is embraced and comprised within it by reason of 

 the direction of the development of the polypary. It is this, which 

 corresponds to the thi-ead-like pi'olongation of the apex of the sicula 

 mentioned above, which occurs in certain species of Didymograptus, 

 e.g. D. gibberulus, Nich., where it extends beyond the length of the 

 branches. 



Since the apex of the sicula is free, as is the case in the above-cited 

 Graptolite forms, a virgula cannot consequently occur embedded 

 in the dorsal side of the branches. It may be observed that in 

 genera in which the sicula is embedded in the polypary a virgula 

 need not of necessity be present. 



The presence or the absence of a virgula depends on whether the 

 initial end of the sicula is protracted or not. Thus, for example, the 

 virgula is absent in PJiyllograptus, and such has hitherto never been 

 observed in any allied species ; and in more than a hundred of my 

 isolated or otherwise prepared examiples of Fhyllograptus angustifolius, 

 Hall, in which the internal structure can be studied, no trace of 

 a virgula has ever been observed. 



The second of the forms referred to the virgula by "Wiman 

 consists of a cylindrical chitinous thread, which originates as 

 a result of growth within the apertural end of the sicula, at some 

 distance from the initial portion. This later structure stands evi- 

 dently in no relation whatever to the real virgula, but may 

 be regarded as an apertural spine. 



The presence of a virgula has, curiously enough, hitherto been 

 considered as the main character of Graptolites. Thus, for example, 

 Lapworth ^ characterizes the sub-order Rhabdophora as follows : 

 " Hydroida in which the polj'^pary is strengthened by a chitinous 

 filiform virgula." Zittel '^ also emphasizes the presence of a virgula 

 in the sub-order Graptoloidea, and says : " Through the possession of 

 a virgula, Graptolites differ from all other Hydroida." The generally 



1 Geol. Mag., Vol. X, 1873, p. 555. 



2 Handb. der Pal., p. 293. 



