318 BRITISH GRAPTOLITES. 



.[(Unities. — lletiog. aculeatus (which Lapworth made the type of his genus 

 Idiogra/ptus) resembles lletiog. Geinitzianus in its small size, in the tenuity of the 

 periderm and the development of the clathria. It is, however, altogether a more 

 lax form and the apertural spines are more conspicuous ; in one example, 

 indeed, some of these appear to be long and flexed and suggestive of those of 

 the Lasiograpti. 



So far as can be gathered from the published figures and descriptions, this 

 form appears to be closely allied to Hall's American species — Betiograptus eucharis 

 (' Graph of Quebec Group,' dec. 2, 1865, pi. xiv, fig. 9) — made classic by the 

 researches and observations of Ruedemann (' Grapt. of New York,' pt. 2, p. 397 

 et seq., pi. xxvi, fig. 19; pi. xxvii, figs. 11 — 13, etc.). 



Horizon and Localities.- — Middle Bala, Middle Ardmillan Series, and Hartfell 

 Shales. 



8. Scotland •• Shalloch Mill, Girvan ; Dobb's Linn ; Syart Law. 



Associates, etc. — Retiog. aculeatus is common in a single bed in the Middle 

 Ardmillan Series of Shalloch Mill, Girvan ; it is rare in the Hartfell Shales of 

 Dobb's Linn and Syart Law. 



Collection. — Lapworth. 



Genus LASIOGRAPTUS, Lapworth (extended). 



1873. Lasiogra/ptus, Lapworth, Geol. Mag., vol. x, p. 559. 



Polypary bilaterally symmetrical, quadrangular to concavo-convex in transverse 

 section, with sub-parallel margins. Septal and ventral lists conspicuous, 

 appendages in the form of spines, or of fibrous processes which may 

 (1) remain free, (2) may be connected by a web, or (3) may anastomose 

 to form a more or less complete marginal lace-work. 

 Thecse of the general type of those of Amplexograptus. 



The tendency to special localisation of thickening in the periderm, which first 

 becomes conspicuous in the Olossograpti, and is intensified in the Itetiograpti, 

 is developed to a remarkably high degree in the Lasiograpti. The edges and 

 angles of the thecee become differentiated almost into a continuous cord, filament, 

 or fibre, that shows through the test along the thickened selvages like a rib in an 

 umbrella or the piping of a hem. 



Broadly speaking, it may be said that it is this listed and fibrated structure 

 which is continued into, and constitutes the main element of, the extraneous 

 processes generally, which are more varied in form in the Lasiograpti than in any 

 other known group of biserial Graptolites. 



