812 DR P. R. C. REED ON THE 



passing imperceptibly into broadly rounded anterior margin. Brachial valve with 

 more obtuse beak. Interior of brachial ? valve with thin median septum about half 

 the length of valve. Surface of shell ornamented with rather irregular sublamellose 

 concentric ridges radially fimbriated or armed with short recumbent spines, especially 

 near margin. 



Dimensions. — Length, 3 "2 mm. ; width, 3'0 mm. 



Horizon. — Drummuck Group. 



Locality. — Thraive Glen. 



Remarks. — This small imperfectly known species, of which the largest example 

 only measures 5 or G mm. in length, seems to belong to the genus Siphonotreta, 

 judging from its resemblance to S. micula, M'Coy. 



Genus Schizambon, Walcott. 



Schizambon scoticus (Davidson). 



(Plate III, figs. 9-14.) 



1877. Siphonotreta scotica, Davidson, Geol. Mag., vol. iv, 1877, p. 13, pi. ii, figs. 5, 6. 



1883. Siphonotreta scotica, Davidson, Mon. Brit. Foss. Brach., vol. v, Silur. Suppl., p. 217, pi. xvi, 



figs. 31-33. 

 1883. Lingula Ramsayi, Salter, Davidson (pars) ibid., pi. xvii, fig. 6. 



Shell broadly oval. Pedicle-valve with subacuminate terminal beak, shoulders 

 enclosing angle of about 120°; anterior end rounded; subcentral oval foramen, with 

 external rounded groove running back from it to beak and tapering posteriorly. 

 Interior of pedicle-valve showing pedicle-groove as a low broad rounded ridge 

 slightly widening and thickening from beak to subcentral foramen. Brachial valve 

 gently convex, with blunter, more obtuse beak ; narrow weak median longitudinal 

 depression traversing surface and dying out near anterior margin. Surface of both 

 valves covered with numerous concentric sublamellose imbricating ridges with finer 

 concentric lines between ; from the ridges arise widely separated scattered short 

 tubular spines or hollow tubercles usually arranged alternately on successive con- 

 centric ridges in an irregular quincunx manner ; nearer the edges of the valves the 

 spines are longer, and closer together, and round the margin a horizontal fringe of 

 long straight radiating spines of subequal or equal strength projects nearly one-eighth 

 the length of the shell. Substance of shell minutely punctate. 



Dimensions. — 



Length .... 



Width .... 

 Horizon. — Stinchar Limestone Group. 

 Localities. — Craighead, Minuntion. 



Remarks. — Owing to Davidson's diagnosis of the species being imperfect and 

 incorrect, the preceding revised description has been drawn up from a re-examination 



(1) 



(2) 



15'0 



c. 20"0 mm 



13'0 



16'0 mm 



