lii BRITISH GRAPTOLITES. 



stipe, which existed as one of the members of the entire frond, the true initial point 

 of which would be in the centre of the whole." It is clear from Hall's observations 

 that the term was not in all cases given by him to one and the same structure, 

 sometimes being identical with what is now known as the sicula (Coenograptus), 

 sometimes to the apical part of the sicula (Didyinograptus), and sometimes to the 

 " apertural spine" of the sicula (Diplograptus). Hall emphasises, however, one 

 important fact, now known to be true of the sicula, viz. that the radicle passes into 

 the " commencement of the solid axis." 



Funicle. — Hall believes that in the Graptolites with four stipes (Tetragraptus) 

 " the condition appears like that of two individuals of the two-stiped forms con- 

 joined by a straight connecting process of greater or less extent, destitute of 

 cellules," and this he calls the " funicle." The greater the number of stipes, the 

 greater are the number of the divisions of this non-celluliferous funicle. (It has 

 subsequently been shown by more recent observers that the non-celluliferous 

 character of the dividing and subdividing primary and secondary stipes is only 

 apparent, being merely due to their mode of preservation. Consequently Hall's 

 funicle as originally defined by him is non-existent.) 



Central Disc. — This curious structure, first recognised and described by Hall, 

 " appears to be composed of two laminae which, at least in the central parts, are 

 not conjoined, and the space is probably occupied by some softer portion of the 

 animal body." According to Hall, the functions of this disc are to " give strength 

 and support to the bases of the stipes," but beyond this " it probably serves other 

 purposes of the animal economy," and he seems to hint that reproduction was one 

 of these (loc. cit., p. 35). 



Solid Axis. — Hall adds but little in this work to our knowledge of the " solid 

 axis," merely corroborating Barrande's observations ; but he recognises its import- 

 ance in the Graptolite structure as a whole, regarding it as the " foundation on 

 which the other parts are erected." Although believing the axis to be solid, he 

 distinctly acknowledges in a note that " the aspect of the axis, when marked by a 

 longitudinal groove, is precisely that which a hollow cylindrical body would have if 

 extremely compressed" (p. 22). 



Common Canal. — While recognising the fact that the common canal gives rise 

 to the cellules in most cases, Hall points out that in such forms as Or. gracilis it 

 must also give origin to " simple small stipes with solid axis, common canal, and 

 cellules." He lays great stress on this double function of the common canal, but 

 unfortunately his view of this question is largely due to his erroneous idea of 

 the non-celluliferous character of the main stipe in Or. gracilis. In the case 

 of Diplograptus, Hall considers that there may be either two common canals 

 separated by the "axis becoming a flattened plate," or else a single canal with 

 only a filiform axis. He did not therefore distinguish between the septal walls 

 between which the axis rims, and the axis itself. In Yhyllograptus he infers that 



