256 



BRITISH GRAPTOLITES. 



tus, Nich. 



1 



subsequently, more gradually till a maximum of about 2 mm. is attained. 

 Sicula small, virgella conspicuous, but rarely preserved. Theca? twelve to 

 ten in 10 mm., with marked sigmoid curvature and sliglit torsion, free edge 

 rounded, length 15 mm., overlapping one half their length; apertural 

 margins slightly undulate and everted in bi-profile view. 

 Description. — The polypary presents a very characteristic appearance due to 

 the peculiar mode of growth of the theca?. Each of these grows at first in a 

 Figs.175 a and b.— Glyptograptus sinua- normal manner parallel with the general direction 



of the polypary, but then takes a curve outward, 

 and the axis twists so that the theca in its apertural 

 region is turned at an angle to its previous direction 

 of growth. The theca in its general form strongly 

 recalls that of certain Dicellograpti. 



The polypary as a rule does not exceed 1 mm. 

 in breadth ( - 7 mm. in relief) for the first 3 mm. of its 

 extent; then, however, it abruptly widens to 1*5 mm. 

 and subsequently very gradually till a maximum 

 width of about 2 mm. is attained. In a feAv speci- 

 mens the increase of breadth appears to be gradual 

 throughout the proximal region of the polypary. 

 The visible portion of the sicula is barely 1 mm. 

 long, and the virgella when preserved may have 

 a length of 5 mm., but this is exceptional (Fig. 

 175^). 



In the slender proximal region of the polypary 

 the theca? are markedly alternate in their arrange- 

 ment ; there are four pairs of theca? in this narrow part, and in profile view the 

 apertural margins tend to appear slightly everted. In the obverse aspect the 

 aperture of th. I 1 does not reach to the line of the apex of the sicula, while that of 

 th. \- is distinctly above it, and the sicula is free for a considerable fraction of its 

 length on one side. The septum seems to originate at once. 



The overlap of the theca? is least in the narrow proximal region, being 

 barely one third of the thecal length, whereas in the wider part of the polypary an 

 overlap of fully one half their length is attained. In the reverse aspect the 

 peculiar mode of growth of the theca? causes their apertural margins to occupy 

 fully one quarter the breadth of the polypary. 



Affinities. — The only Glyptograptus to which G. sinuatus is closely allied is 

 G. persculptus, from which, however, it differs in point of size and in having 

 closer set theca?, which are also less twisted. From all other Glyptograpti at 

 present known the peculiar mode of growth of the theca? should be sufficient to 

 distinguish it. 



a. Proximal end in relief, showing long 



virgella. Skelgill, Ambleside; Skel- 

 gill Shales. Coll. Brit. Museum 

 (Nat. Hist.). 



b. Distal theca' in relief. Enlargement 



of part of PI. XXXI, fig. 6 a. 



