108 BRITISH GRAPTOLITBS. 



the zone of Pleurog. linearis, and is often found in the zone of Dicranog. Glingani. 

 It is commonly associated with L&ptog. ccupillaris, Dvplog. truncatus, var., and other 

 Diplogra/pti of the "foliaceus" type. 



Good specimens are in the collections of Lapworth, the Geological Survey of 

 Scotland, and the Woodwardian (Sedgwick) Museum. 



Note. — L.flaccidus has a fairly large number of varieties distinguished by their 

 general habit, number of thecae in a given unit of length, and the character of the 

 proximal end. Most of these varieties, as well as L. flaccidus itself, in addition to 

 a two-stiped form, present us with occasional mutations in which an extra stipe or 

 branch is present. This stipe originates from the sicula or its immediate neigh- 

 bourhood, and may be either simple or compound. 



Fm. 62 g.-Leptograptes flaccidus (Rati). -.-x , i 



In these centribracniate mutations we nave been 



unable to determine with certainty how this third 



stipe arises. Sometimes it looks as if it were 



'-~-gc~l merely the prolongation of the apical part of the 



CentribracWate form. Enlargement of sicula, but in other specimens the apex of the 



part of PI. XIV, fig. 1 g. ' . L \ 



sicula is clearly visible. It seems possible that it 

 is an abnormal growth from the initial bud from which the two normal stipes of 

 the polypary are developed, or in other words, it is an extra division of the 

 crossing canal. Thus the branching, instead of being deferred to a late stage 

 in the growth of the polypary, as in the Plearogrcupti, is concentrated in its early 

 stages. 



Var. spinifer, var. nov. Plate XIV, figs. 2 a — c. 



Description. — In addition to L. flaccidus, there is found on approximately the 

 same horizon, a somewhat closely allied form, which differs from the typical species 

 in the mode of origin of its stipes, its more irregular curvature, its longer sicula, 

 ;nid the more conspicuous spines on the proximal thecas. As regards the size of 

 the stipes and their increase in width, this variety agrees closely with the typical 

 species. 



The curvature of the stipes is very variable : at first and for some little 

 distance, it is occasionally markedly convex with respect to the ventral margins, 

 far more so than in L. flaccidus; it may then become slightly concave, and 

 ultimately the stipes appear to run in an approximately horizontal direction; in 

 some specimens, however, the stipes are scarcely curved at all. They widen 

 gradually and persistently throughout their length from '4 mm. (exclusive of 

 spines) near the sicula, to '8 mm. near the distal end ; in very long forms a breadth 

 of 1 mm. may he attained, but the smaller narrower forms seem to be of commoner 

 occurrence. 



