The GraptoUtes of Lance field Beds. 167 



Didymogpaptus, McCoy, 1851. 



DiDYMOGRAPTUS PRITCHARDI, n. sp. 

 (PI. XVIL, Figs. 7, 9; PI. XIX., Figs. 8, 10). 



Hydrosome very slender, the two branches at first forming an 

 angle of about 140° with one another, then gently and evenly 

 curving, with the concavity on the theca bearing side. Not 

 infrequently a third branch is given off, from the neighbourhood 

 of the second theca, while rarely another branch arises from a 

 similar position on the other side of the sicula. Branches about 

 0-5 mm. wide, and may reach a length of over 12 cm, Thecae 

 about 10 in 10 mm., very long and narrow, overlapping appar- 

 ently about one half their length ; apertural margin straight or 

 slightly convex, at right angles to the branch. Outer margin at 

 first making a very small angle with the branch, but towards the 

 aperture becoming deeply concave and foi'ming an acute point. 

 Virgula at times extending for a considerable distance from the 

 proximal end of the sicula. 



This slender form is fairly common and almost always has but 

 two branches developed, so that I have ventured to refer it to 

 Didymograptus. Had the four branched forms been at all 

 common, instead of rare, they might have been regarded as the 

 normal ones, and those with two or three branches as having 

 arisen by the suppression of one or more branches. The 

 exceedingly narrow proximal part of the thecae led me at first 

 to think that no overlap took place, and that the form was a 

 Leptograptid, but that the thecae do overlap is clear from the 

 examination of a specimen preserved partly in relief. The 

 virgula is at times visible as an exceedingly fine thread and may 

 reach the length of 4 mm. 



Didymograptus taylori, n. sp. 



(PL XVII., Figs. 11, 12). 



Branches diverging at an angle of from 140° to 160°, straight, 

 0-5 mm. broad near their origin and gradually increasing to 

 0-75 mm. at their distal end, each from about 5 to 10 mm. long. 

 Sicula about 1 d mm. long. Virgula extending for some 3 or 4 



