224 



BRITISH CtRAPTOLTTES. 



Fins. 11-.") r, (?, c,/'. — Orthoi/raptus quail ri 

 mucronatus (Hall). 



to one vertical row on each side are shown to their full length, and the edge 

 of the apertural margin is exhibited almost to its full extent, occupying nearly 

 one half the breadth of the body of the polypary. The commonest view, however, 



is the bi-profile (Fig. 146), in which the lobation 

 of the apertural edge causes the visible part of the 

 apertural margin to appear sometimes broadly 

 rounded or notched, while the apertural spines are 

 greatly foreshortened or quite invisible. What 

 seems to be a variation of this view is some- 

 times presented when the foreshortened apertural 

 margin appears as if inclined, and the spine is, as 

 a rule, invisible (Fig. 145/). The apertural spines 

 are stout, and may measure as much as 1*5 or 

 2 mm. in length ; their apparent direction is, how- 

 ever, varied by compression, and may appear 





1 N. 



. <> : r 



f 



horizontal, downward or upward. 



The basal thecas, which are small, bear, in 

 addition to the usual pair of apertural spines, a 

 short (? mesial) spine or spur, which is, however, 

 very rarely shown (Fig. 145 b). 



Affinities. — 0. quadrimucronatus is related in 

 form to the other spinose members of the group, 

 such as 0. Whitfieldi, from which it differs in the 

 form of the polypary, and in the length and direction 

 of the spines. It also presents certain close resem- 

 blances to the forms grouped in the family of the 

 Glossograptidas. 



Horizon and Localities. — Hartfell Shales (Pleurog. 

 linearis zone). 



8. Scotland : Dobb's Linn ; Hartfell ; Mount 

 Benger Burn, etc 

 Associates, etc. 



an uncommon fossil in the Hartfell Shales of S. Scotland, where it occurs in 

 the zone of Pleurog. linearis associated with the zone fossil, and with Orthog. 

 truncatus, var. pauperatus, and other forms. The specimens from Mount Benger 

 Burn are generally narrower than those from other localities, and the theca? are 

 somewhat more closely set. 



Collections. — Lapworth, British Museum (Natural History), Geological Survey 

 of Scotland, and the Authors. 



c. Distal thecse. Sub-soalariform view, 



showing full length of apertural 

 spines. Enlargement of part of 

 PI. XXVIII, fig. 1 a. 



d. Sub-scalariform view, showing varia- 



tions in apparent length and position 

 of apertural spines. Mount Benger 

 Burn, S. Scotland, Hartfell Shales. 

 Coll. Lapworth. 



e. Sub-scalariform view, showing further 



variations in the shape of the aper- 

 tural margin. On same slab as 

 Fig. 145 d. 



f. Bi-profile view, in which apertural 



margin appears inclined, and spines 

 are almost invisible. Enlargement 

 of part of PI. XXVIII, fig. 1 c. 



■Orthog. quadrimucronatus is not 



