LEPTOGRAPTUS. 



107 



Figs. 62 c and d. — Lvptograptus flaccidx 

 (Hall). 



a - -. 





c. Proximal theca;. On same slab as PL 



XIV, figs. 1 c, d. 



d. Distal thecse. Enlargement of^part of 



specimen on 

 fig. 1 b. 



of a second crossing canal (c.c. 2 ) somewhat similar in character to that observed 

 in the case of Didymograptus gibberulus; th. 2 1 appears to be developed Prom the 

 initial part of th. I 2 , and th. 2 2 also from th. I 2 . This is a matter of special 

 interest, because this type of development is known also in the Dicranograptidse 

 and the Diplograptidse. 



The two earliest thecas are only about half as long- as those subsequently 

 developed, and their apertures are conspicuously mucronate, but not spinose, a 

 feature which is shared by th. 2', th. 3 1 , th. 2 2 , and th. 3 2 only. 



The average length of a theca in the maturer 

 parts of a stipe is about 2 mm. ; the width of each 

 theca is uniform throughout its length, and 

 averages "2 — '3 mm. ; each thecal aperture 

 occupies half the width of the stipe in the 

 proximal region, but distally the proportional 

 fraction is much less (one third). The thecas 

 average nine to eight in 10 mm. distally, but they 

 are rather more closely set near the sicula, being 

 slab as pi. xiv, commonly about eleven in 10 mm. ; their inclina- 

 tion is uniformly low, and the overlap is usually 

 about one-half their length, but may be rather less. 



Remarks. — The British specimens of L. flaccidus agree very closely with the 

 American forms from the Utica Slates. Hall gives 

 Figs. 62 e a^df.-Leptogrwptus flaccid™ a higher number of thecse in a given unit of length, 

 6 but possibly he adopted a different method of 



measurement, for the American specimens in the 

 collection of the Canadian Geological Survey, which 

 we have examined, agree precisely with the British 

 specimens in this respect. 



Affinities. — L. flaccidus is characterised by its 

 curvature, by the gradual and persistent widening 

 of the stipes for the first few centimetres of their 

 extent, and by the number of theca) in a given unit 

 of length. It is distinguished from its varieties by 

 its general form and the character of the proximal end. 

 Horizon and Localities. — Lower Hartfell Shales. 



8. Scotland: Mount Bengcr Burn; Hartfell Spa ; Syarl Law Scaur; Craig- 

 michan Scaur; Berrybush Burn; Stinchar Valley; Marl Hill ; ScabCleuch; Yellow 

 Mire; Penwhapple Burn. N. Ireland : Bally grot ; Tramore Bridge. N. Wales: 

 Derwendeg, near Conway. 



Associates, etc. — L. flaccidus occurs in profusion in many Scotch Localities where 

 the lower zones of the Hartfell Shales are well exposed. It is a common fossil in 



e. Specimen from Hall's original lo- 

 cality, nat. size, for comparison 

 with British examples. Utica 

 Slates, Lake St. John. Coll. Cana- 

 dian Geological Survey. 



/. Enlargement of thecee of Fig. 62 c. 



