Tlie Graptolites of Lancefield Beds. 171 



visible. Primary branches in a straight line each about 1 mm. 

 long. Secondary branches forming an angle of about 90° with 

 each other, about 2 or 3 mm. long. Tertiary branches diverging 

 at about 65°, and 4 or 5 mm. long. Those of the fourth order 

 branching at 50°. Thecae narrow, apertural margin nearly at 

 right angles to the branch ; outer margin straight or slightly 

 concave, inclined at an angle of about 10°; 10 or 11 present in 10 

 mm., and present on secondary and all succeeding branches, but 

 not observed on the primary. 



The above description is drawn up from specimens before me 

 and agrees well with the very full one given by Moberg of the 

 Swedish forms. Linnarsson in his original description gives 7-8 

 thecae as the number in 10 mm., as also does Frech,^ who 

 had well preserved specimens from the type locality. Moberg 

 however gives 8-10 as the number in the same length, while 

 the Lancefield specimen seem to have 11 pretty constantly. 



If the specimens I have identified as C. rigidus and C. tenellus 

 are correctly determined, and to my mind there is no doubt that 

 they are, then there can be no reasonable ground for their specific 

 separation. Were extreme forms alone considered their union 

 under one specific title would be doubtful, but the occurrence 

 of a large number of intermediate examples, which one hesitates 

 to refer to either " species," compels us to unite the whole series 

 under the name of the first described form, C. rigidus. The 

 thecae of both C. rigidus and C. tenellus are similar in shape, 

 about the same number occur in a given space in both forms, 

 while the general appearance of the hydrosome differs only in 

 the smaller build of Linnarsson's species and in its less copious 

 branching. That this is subject to considerable variation the 

 diagrams below will make clear. In drawing them up I have 

 merely measured with dividers and a boxwood scale to the 

 nearest half millimetre. The measurements in each case will 

 be seen to be aggregated round one common mean, and not two, 

 as would be the case if the length of the branches afforded a 

 satisfactory means of separation. Each dot represents a single 

 measurement, 



1 Leth. Geog-. iTh. IB., p. o98, 1897. 



