204 TWENTIETH REPORT ON THE STATE CABINET. 



existed in compound fronds, having their origin from a central point^not 

 ■unlike in the commencement to Graptolithus logani, but without the 

 central disc. These fronds were probably concavo-convex, as were the 

 individual stipes. The solid axis, instead of being central, is placed exter- 

 nally along the centre of the convex or outer side ; and the cell-divisions 

 on that side reach to and join it ; while on the upper or concave side the 

 cell-divisions do not reach the centre, leaving a space for the common 

 body, which has been shown by Barrande and Geinitz to produce a 

 central longitudinal prominence. 



In these forms the mode of development has been similar to that already 

 explained, the modification being chiefly in the external position of the 

 axis and the joining of cell-divisions with the axis on one side ; leaving 

 the common body in a somewhat triangular form, from which the alveoles 

 are developed on either side. 



Whether the Phyllograptus existed as simple stipes with four ranges of 

 cellules, or in a compound arrangement as in Retiograptus, the mode 

 of development has been similar : either the germ with its initial point 

 developed a single stipe with four ranges of cellules, or the same elements 

 first subdivided, and each division gave origin to its stipe through the 

 common body. 



In regard to the development of the cellules in the difi"erent parts of 

 the graptolite, we observe, as a uniform feature, their lesser development 

 towards the base of the stipe. In all the monoprionidian forms, this cha- 

 racter is particularly observable ; and in a few species, the earlier cellules 

 are raised in a scarcely perceptible elevation above the general surface of 

 the common envelope. Indeed, in a few instances, it is impossible to ascer- 

 tain satisfactorily whether these earlier prominences are expanded into 

 open cellules. As the stipe is extended, they become gradually more and 

 more prominent, until towards the middle, or oftener perhaps nearer the 

 distal extremity, their greatest degree of development is reached. In 

 some species this takes place near the base, and in the more elongated 

 stipes there is no sensible increase throughout a great part of their length, 

 and the two margins of the stipe are essentially parallel. Towards the 

 distal extremity there is a gradual, or often a more abrupt, diminution in 

 the size of the cellules ; and a few of the last ones are much smaller, until 

 the terminal cellule is sometimes seen in a partially developed condition 

 between the common body and the partition of the preceding cellule. 



The same condition of development in the cellules is true of the diprio- 

 nidian forms, as is shown in some degree in G. pristinifor?nis, Plate xiii 

 (Canadian Decade, ii), but more especially in the accompanying figures 

 of G. pristis and G. whitfieldi, the last of which also shows the higher 

 cellules diminished, so as to contract the width of the stipe above (figs. 

 32 and 33, p. 205). 



