66 



BRITISH GRAPTOLITBS. 



Fios. 40 a and h. — Tctragrnptus serra, Brono-niart. 



-i^^'^^i^ 



a. Proximal end, obverse view, preserved partly as 

 an impression, showing the dividing wall 

 originating in th. li. Bennane Head, Bal- 

 lantrae. Coll. Lapworth. 



6. Proximal end, showing part of nema. Outer- 

 side. Coll. British Museum (Natural His- 

 tory), S. Kensington. 



orig-in, but widen rapidly, and avc o-cnovally found to liaA^e attained tlieir maximnm 

 widtli after the third or fourth theca; the usual diminution takes place towards the 

 distal extremities. The width attained in different individuals varies greatly; the 

 average width at origin is about 1 mm. (or less), but while in some individuals a 

 width of 2'] mm. is never exceeded, in others a maximum of 3"2 mm. is attained. 



The sicula is rather more than 2 mm. in 

 length, and is proportionately narrow. The 

 first theca appears to originate near the apex 

 ''Nw^ Jk ^ \ ifl A, o^ ^^it' sicula, in a manner somcAvhat resem- 



ri^ -.- ^ ,\h.^ ^V'^.x fi J^i. |,^j^^g. ^1^^^^ Q^ jj gibherulus ; but other details 



are obscure. Both the earliest thecffi (th. 1^ 

 and th. 1^ forming the stipes of the first 

 order) are small as compared with those 

 subsequently developed on the main stipes, 

 and dichotomous division takes place within 

 them. 



The thecEe on the main stipes vary in 

 number according to their position on the stipe, and the size and age of an 

 individual. Near the proximal end of the main stipes there may be ten in 10 mm., 

 but distally the number rarely exceeds eight. AYhen fully developed the thecos are 

 about 3"2 mm. in length; they are somewhat curved, and are widest at their 



apertures. The inclination varies from 30° to 50° 

 owing to curvature, the thecce being inclined at 30° 

 in their central regions, but then curving upward 

 so as to make, near their apertures, an angle of 

 from 45° to 50° with the general direction of the 

 stipe. In some specimens the growth-lines are 

 well seen. 



Remarks. — Hall's descriptions of his G. hryonoides seem to have been drawn up 

 from two forms — one in Avliich the stipes are directed obliquely upward, and one in 

 which the stipes are spread out in what may be termed a ^' guadrihracltidins" 

 fashion. This implies two different modes of growth, and there are also other 

 differences. It is therefore necessary to distinguish between the two forms ; 

 the one with the stipes directed obliquely upward is the form which Hall con- 

 sidered to be identical with Tetrngraptus serra, an opinion in which we agree. The 

 other is that which is here designated T. Amii. 



Certain examples of Tetragraptus from S. Wales, described by Hopkinson 

 ('Quart. Journ. Geol. Soc.,' xxxi, pi. xxxiii, fig. 11) as T. Ealll, Ave believe to be 

 greatly compressed specimens of 2'. serra. 



Affinities. — T. serra, as here restricted, is readily recognisal)le ; it can hardly be 

 said to closely resemble any other species as yet known, and should always be 



Fia. dOc. — Tdragraptus serra, Bron^ 



Thecse in relief, showing growth-lines. 

 Bennane Head, Ballantrae. Coll. 

 Lapworth. 



