:] 12 



BRITISH GRAPTOLITES. 



Retiolites (Plegmatograptus) obesus (Lapworth). Plate XXXIV, figs. 12a— c. 



1870. Retiolites perlatus, Lapworth, Cat. West. Scott. Foss., pi. iii, fig. 61. 



1877. Retiolites perlatus, var. obesus, Lapworth, G-rapt. Co. Down, Proc. Belf. Nat. Field Club, p. 137, 



pi. vi, fig. 29. 

 1890. Reliolites obesus, Tornquist, Acta Univ. Lund., vol. xxvi, p. 10, pi. ii, figs. 24, 25. 



Polypary 2 — 3 cm. in leng 



Figs. 223 a, b, and c. — Plegmatograptus 

 obesus (Lapw.). 



<i% - 



i 





SI ! 



. 



a. Part of specimen showing ventral 



lacinia and texture of retieula. 

 Elwand water, near Melrose ; Gala 

 Group. Coll. Lapworth. 



b. Bi-profile view. Enlargement of part 



of PI. XXXIV, fig. 12 a. 



c. Scalariform view. Enlargement of 



part of PI. XXXIV, fig. 12 c. 



tb, and relatively broad ; having an average uniform 

 breadth of about 8 mm. attained by rapid 

 expansion in the proximal region. Thecse 

 with indistinct outlines, twelve to nine in 

 10 mm.; apertural margins concave, undulate; 

 outer margin formed of a ventral braid 

 apparently composed of paired inosculating 

 fibres. Elements of clathria rarely show- 

 ing; retieula filamentous with sub-hexagonal 

 meshes. 

 Description. — The polypary is somewhat small, 

 but relatively very broad ; the initial width does 

 not exceed 1*5 mm., but increase takes place so 

 rapidly that the maximum width is quickly attained, 

 and the margins are then parallel for the remainder 

 of their extent. 



What appear to be the denticles of the thecas 

 are occasionally well defined and thickened ; they 

 appear to be either concave or undulate. The 

 ventral margin of the fossil is bordered by a broad 

 meshwork of curved threads which form a com- 

 plete marginal braiding. The main fibres of the 

 meshes appear to arise from the angles of the aper- 

 tural margins and are paired. With the exception 

 of its straight septal strand the clathria is ill- 

 defined. The retieula, however, is usually well 

 preserved, and is composed of irregular and some- 

 what wide sub-hexagonal meshes of fine thread-like 

 fibres. 



Affinities. — P. obesus can readily be distinguished 

 from Plegmatog. nebula by the more robust char- 

 acter of the polypary, and the elegant and character- 

 istic shape of the regular ventral meshes. 

 Horizon and Localities. — Gala-Tarannon. 

 S. Scotland : Elwand Water, near Melrose ; 



