LEPTOGRAPTUS. L05 



makes the accurate determination of its length a matter of some difficulty, since the 

 apex merges gradually into the nema. The neiiia is, however, rarely preserved for 

 any length, no examples being known in which it exceeds 5 or G mm. 



The general lines of development of the polypary are the same as those 

 already described in the case of Didymograptus, but the presence of what may be 

 termed two crowing canals instead of one indicates in this genus a special phase 

 of development. The presence of these canals connecting the primary and 

 secondary stipes is usually indicated in the reverse aspect by a thickening in 

 the dorsal region of the proximal end. It is obvious in the more robust forms, 



and in the rare specimens preserved in relief, but 



Fia. 61.— Lcptograi>tuscf.jlaccidns {Hull). . ., ,.„ , „ -. . . , 



is naturally .difficult 01 detection m the very 



JL j^gqg slender and compressed specimens. Nevertheless 



"- ^y' W e believe that these two crossing canals are 



Reverse view showing the thickening present i n every case and are to be regarded as 



produced l>y the two crossine; canals, r J o 



Haxtfeii Spa, HartfeU shales. Coll. one f the characteristic - features of the genus 



Lap worth. ° 



Leptograptus. 



The thecal apertures are inclined with respect to the general ventral margin of 

 the polypary and are slightly introverted. In front of the aperture, the ventral Avail 

 of the succeeding theca has the aspect of having been pressed inwards, thus giving 

 origin to a kind of " excavation " in the general ventral margin of the polypary 

 itself, and the aperture opens partly within this excavation. The ventral margin 

 of each individual theca has a slightly sigmoid curvature, and this character with 

 the "excavation" and inclined aperture is very characteristic not only of the 

 Leptograpti, but of the family as a whole. 



It is not possible in the present state of our knowledge to divide the Leptograpti 

 into any definite serial groups, so intimately are they related to one another in their 

 habit of growth. Extreme forms are no doubt readily distinguishable, but they 

 are connected by a series of forms which grade so insensibly into each other that 

 it is impossible to draw a divisional line between them. Thus a flexed form like 

 //. flaccidus, by gradual increase in the amount of flexure, passes on the one hand 

 into a form with a sigmoid habit such as L. capillaris ; and on the other hand, by 

 decrease in the amount of curvature of the stipes, into a more or less straight 

 form like L. latus or L. gramdis, and yet the sigmoid, the flexed, and the relatively 

 straight forms are obviously distinct. 



The following species are at present included in the genus Leptograptus : — L. 

 flaccidus; L. flaccidus, var. spinifer ; L. flaccidus, var. macilentus ; L. flaccidus, var. 

 macer ; L. flaccidus, var. arcuatus ; L. capillaris ; L. validus; L. validus^&r. incisus ; 

 L. grandis ; L. latus ; L. ascendens. 



