164 BRITISH GRAPTOLITES. 



Genus DICRANOGRAPTUS, Hall. 



1865. Climacograptus (Dicranograptus s — g.), Hall, Grapt. of Quebec Group, p. 112. 

 1868. Dicranogr aphis, Hall, 20th Kept, on State Cabinet, p. 218. 



Polypary bilaterally symmetrical, consisting of a proximal biserial portion, 

 dividing distally into two uniserial stipes with an axillary angle of less 

 than 90°. 



Thecse of the type characteristic of the family. 



The polypary in Dicranograptus varies much in size in the different species, and 

 also in the relative lengths of the biserial and uniserial portions. Most of the 

 species are characteristically robust, though a. few are comparatively slender. 



The sicula is small and is rarely visible, being as a general rule more or less 

 completely embedded in the polypary. It is seen only in the obverse aspect, being 

 completely concealed in the reverse aspect by the growth of the proximal thecse. 

 Its apertural spine (virgella), however, can be frequently detected. The "lateral" 

 spines so often visible at the proximal extremity of the polypary are of a precisely 

 similar nature to those in Dicellograptus. 



The development of the biserial portion of the polypary (the " stem" of some 

 authors) affords some features of interest. Th. I 1 develops on the left of the 

 sicula (obverse aspect), grows at once partly upward, and partly outward, for a 

 short distance, then bends somewhat abruptly and becomes more or less vertical 

 for the remainder of its length. Th. I 2 is developed from the initial end of th. I 1 , 

 and grows in a similar manner, but towards the other side of the sicula. Thus 

 the " crossing canal " is theoretically reduced to a minimum and as such may be 

 said to be practically non-existent. Th. 2 1 is developed from the initial part of 

 th. I 2 and its aperture lies immediately over that of th. I 1 . 



In some forms the alternate arrangement of these four primary thecse thus 

 brought about seems to persist throughout the whole of the biserial portion, so that 

 we have the appearance of a double series of thecse with a single " common canal." 

 In most species, however (D. Nicholsoni, etc.), after the growth of a few thecse, a 

 septum is developed, and then each of the succeeding thecse on both sides of this 

 septum grow vertically from the theca immediately below. Thus from the com- 

 mencement of the septum onwards there are two rows of thecse, each with its own 

 " common canal." 



At the termination of the biserial portion the thecse begin to grow in a more 

 oblique direction, the two series become separated in the form of distinct branches, 

 and so continue throughout the remainder of their length. 



Thus in the biserial portion of the polypary of Dicranograptus the type of 

 development is identical with that which rules in the whole of the polypary in the 



