DIPLOGRAPTUS. 233 



remainder of their length. The sicula has a length of 2 nun., and has a slender 

 virgella 1 mm. in extent. Tli. I 1 appears to originate near its aperture and to grow 



at once outward and upward ; from it th. I 3 



Figs. 153 a, b, and c. — Orthograptus mutabilis, nov. -i i • , i 1 i •,] 



develops m the normal manner, and neither 



the aperture of th. I 1 nor that of th. I 2 



reach the level of the apex of the sicula ; 



<C ,7 th. 2 1 develops from th. 1~, but thereafter, 



\ V with the growth of the septum the thecae 



b ' develop in a linear manner. 



f ■' ' jj In profile view the thecce are seen to 



| i/ A i J have a general inclination of about 25°; 



the angle is, however, larger where the 

 polypary is widest, and the thecas more 



a. Obverse aspect, showing portion of sicula. Dobb's Conspicuously Curved Outward. The aper- 



Linn, Birkhill Shales. Coll. Lapworth. , . . -, , , 



b. Reverse aspect ; specimen somewhat distorted, tural margin is everted and somewhat 



giving different appearances in the form of the , t , 



apertural margins. Dobb's Linn, Birkhill Shales rOStl'ate. 



, D^tTtbeiSSarSorn^. ibid. Affinities.-O. mutabilis shows consider- 



able resemblance to the Petalograpti in 

 its proximal region, where the thecEe are long and curved ; the distal part of the 

 polypary, however, is that characteristic of an Orthograptus. From other 

 Orihogra/pti the form of the polypary should be sufficient to distinguish it. 



Horizon and Localities. — Llandovery (Birkhill Shales), Zone of M. gregarius 

 (sub-zone M. cyphus). 



S. Scotland : Dobb's Linn ; Frenchland Burn, etc. 



Associates, etc. — Orthog. mutabilis is particularly abundant at Dobb's Linn in a 

 band at the base of the zone of M. gregarius seen at the head of the Long Burn, 

 where it occurs associated with Monog. gregarius, M. cyphus, M. tenuis, Diplog. 

 tamariscus, 01. rectangularis, CI. Toruquisti, etc. 



Collections. — Lapworth and the Authors. 



Gkoup II. 



Orthograpti in which the thecse were almost semi-circular in section, and the 

 apertural margin plain or but slightly lobate; excavation inconspicuous. 



Diplograptus (Orthograptus) truncatus, Lapw. Plate XXIX, figs. 3 a — e. 



1877. Diplogrwptus truncatus, Lapworth, Grapt. Co. Down, p. 133, pi. vi, fig. 17. 



Polypary characteristically fusiform, 6 cm. or more in length, increasing rapidly 

 in width until maximum of -i mm. is attained ; proximal end broadly rounded. 

 Sicula 2*5 mm. in length, with short but conspicuous virgella, and with 



31 



