DIPLOGRAPTUS. 267 



Horizon and Localities. — Llandovery, Birkliill Shales, Gwastaden Flags (zones 

 of Monog. gregarius and M. convolutus). 



Wales : Rhayader ; River Twymyn, Llanbrynmair ; Rheidol Gorge, below 

 Pont Erwyd ; Llanystmndwy. 8. Scotland : Dobb's Linn, etc. Ireland : Little 

 River, Pomeroy, Co. Tyrone. Lake District : Skelgill. 



Associates, etc. — Mesog. magnus occurs not uncommonly in the Birkliill Shales 

 and their equivalents (zones of Monog. gregarius and Monog. convolutus) ; it is 

 usually associated with Monog. gregarius, M. convolutus, M. fimbriatus, M. jaculum, 

 Glimacog. Hughesi, and Gl. Tornquisti. 



Collections. — H. Lapworth, 0. T. Jones, Fearnsides, and the Authors. 



Geotjp II. 



Mesograpti in which the thecae were semicircular in section and the apertural 

 margins undulate ; excavations deep and conspicuous in the obverse aspect 

 throughout. 



= Amplexograptus. 



Diplograptus (Amplexograptus) perexcavatus, Lapworth. Plate XXXI, figs. 



15 a— d. 



1876. Diplograptus perexcavatus, Lapworth, Cat. West. Scott. Foss., pi. ii, fig. 38. 



1877. Climacograptus perexcavatus, Lapworth, Grapt. Co. Down, p. 140, pi. vi, fig. 35. 



1880. Diplograptus perexcavatus, Lapworth, Ann. Mag. Nat. Hist. [5], vol. v, p. 167, pi. v, fig. 25. 



Polypary short and fairly broad, 1 to 4 cm. in length, with average breadth of 



2 to 2*5 mm. for greater part of length. Sicula 2 mm. in extent, virgella 



conspicuous. Thecce sixteen to twelve in 10 mm., basal ones with short 



spines unsymmetrically disposed ; in the obverse aspect, of the general 



Climacograptus type, with sigmoid curvature of ventral margin, and wide, 



deep and conspicuous excavations occupying one half the breadth of the 



polypary and fully one half its ventral margin ; in the reverse aspect, of 



the general Glyptograptus or Orthograptus type, with less pronounced sigmoid 



curvature of the ventral margin, inconspicuous excavations, and having 



the apertural margins somewhat oblique. 



Description. — The polypary varies greatly in size, but is always fairly robust ; 



the breadth commonly measures 2 to 2*5 mm. ; there is some diminution in width 



towards the proximal end, which has the appearance of being rounded off. 



The sicula appears to have had a length of 2 mm., but its full extent is rarely 

 visible in the adult polypary ; th. I 1 originates some distance up the sicula, 



