258 BRITISH GRAPTOLITES. 



for a considerable fraction of its length on one side. In the reverse aspect it 

 is seen that the septum originates between th. 2 2 and th. 2 1 ; it is slightly sinuous, 

 the sinuosity being more marked in the reverse aspect. 



As usual, the thecse show the most pronounced sigmoid curvature near the 

 proximal end of the polypary, and the torsion of the axis at the curve is also 

 more noticeable in that region. To this is due the peculiar shape of the theca?, 

 which, like those of Glyptog. sinuatus, strongly recall the type of theca in certain 

 Dieellograpti. Apart from their torsion, the thecse appear to be tubes of 

 approximately uniform breadth, having a length of 3 mm., and being overlapped 

 for one half of it. The free edge is straight or slightly rounded in profile 

 view, and has an average inclination of about 15° with respect to the general 

 direction of the polypary. The apertural margin appears slightly concave and 

 everted, and occupies one third to one quarter of the total breadth of the polypary. 



Affinities. — In general form and appearance G. persculptus has many points 

 of resemblance to G. sinuatus, but it is a much larger species, and the thecce 

 are more twisted, and more distant from each other. 



Horizon and Locality. — Lower Llandovery (zone of Gephalog. acuminatus). 



Wales : Gogofau, Carmarthenshire ; Fuches-gan Farm, near Pont Erwyd, 

 Cardiganshire. 



Associates, etc. — The associates of G. per sculpt us at the type locality, Gogofau, 

 are at present unknown, but in the Pont Erwyd district this species occurs with 

 Mesograptus modestus and var. parvulus. 



Collections.— -British Museum (Natural History), Sedgwick Museum, and Mr. 

 0. T. Jones. 



Sub-genus MESOGRAPTUS, nov. 



Within the sub-genus Mesograptus are here included those forms in which the 

 appearances presented are in part those characteristic of a typical Diplograptus 

 (Orthograptus) , and in part those more characteristic of a typical Climacograptus. 



In a first group the earlier thecae have the appearance of those of a Climaco- 

 graptus, with vertical or sub-vertical free edge and well-marked excavation, and the 

 later thecae the appearance of those of a Diplograptus, with inclined free edge and 

 relatively insignificant excavation, and this in both the obverse and reverse 

 aspects of the polypary. Diplog. foliaceus may be regarded as the type species of 

 this group. 



In a second group all the thecae show on the reverse aspect of the polypary 

 the appearances of those of a Diplograptus, and on the obverse aspect those of a 

 Climacograptus. This group, which is typified by Diplog. perexcavatus, has been 

 provisionally separated off by Lapworth as the sub-genus Amplexograptus. 



The flattened forms belonging to the first of these groups are usually easily 



