AMPHIGRAPTUS. 123 



The thecas are very long and narrow, having a length of fully 2 nun. 



Remarks. — Hall notes that there is great irregularity in the length of the 

 stipes, but this would seem to be a matter of preservation. 



Affinities. — A. divergens differs from all other known Amphigrapti in the 

 position and number of the secondary branches; there are usually not fewer than 

 three pairs of these on either side of the sicula, and all branching takes place 

 within a distance of 3 mm. from the sicula, so that there is an appearance of " close 

 packing " of the branches. 



Horizon and Localities. — Hartfell Shales (zone of Pleurog. linearis). 



8. Scotland: Mount Benger Burn; Hartfell Spa; Earl's Hill. 



Associates, etc. — This species seems to be confined to the zone of Pleurog. 

 linearis and is not of common occurrence; it has been found associated with 

 Pleurog. linearis, Leptog. flaccidus, and Olimacog. tubidiferus. 



The figured specimen, and others upon which the description is based, are in 

 Lap worth's collection. The specimen originally figured by Lap worth (' Cat. West. 

 Scott. Foss.,' pi. iii, fig. 70) has unfortunately been mislaid. 



Var. radiatus, Lapworth. Plate XVIII, figs. 2 a — d. 



1876. Arnphigraptw radiatus, Lapworth, Cat. West. Scott. Foss., pi. iii, fig. 71. 



Description. — Another Amphigraptus which is found in the Hartfell Shales, 

 and which appears to be best regarded as a variety of A. divergens, is the one to 

 which the name var. radiatus has been given. 



The polypary of the type specimen is conspicuously radiate in form, being 

 composed of two main stipes and three branches, making five in all ; there is one 

 branch only on each main stipe, one member of each pair being presumably 

 suppressed, and in addition there is a central branch developed from the sicula or 

 its immediate neighbourhood. The form is, in fact, a centribrachiate mutation of 

 one in which there are usually only two main stipes and two branches, making 

 four in all. These two forms, then, are to be considered as constituting one and 

 the same variety, the difference merely lying in the presence or absence of the 

 centribrachiate stipe or branch. 



The main stipes are generally about 1 cm. in length and have a uniform 

 breadth of 1 mm.; the secondary branches agree 

 radiatus, Lapw. Wltn the mam stipes in all essential characteristics, 



and have also a length of about 4 cm. 



The sicula is but rarely preserved; it appears to 



Enlargement of part of PL XV11I, ^ave a length of about 1 '2 111111. 



The thecae number ten in 10 nun.; fchey are 



