146 Frederick Cheq^man .- 



and gently convexly to the anterior margin. The exterior of a 

 ventral valve shows the ornament to be strongly concentric (tegu- 

 late), the edges of the concentric laminae being acerate or sjoarsely 

 spinose. An exfoliated dorsal valve shows the impression of the 

 median septum with divergent furrows and rhomboidal termina- 

 tion ; also muscle scars near the hinge. The interior of the dorsal 

 valve from Lancefield (somewhat distorted) shows the median 

 septum and lateral scars. 



DimenHions. — Ventral valve — length (in all the figured speci- 

 mens from the Mount William district), 2.25 mm. ; width, 2.75 

 mm. Dorsal valve — length, 1.25 mm.; width, 1.5 mm. 



The smaller specimen, from the black shale of Lancefield, has a 

 length of 1.5 mm., and a similar width, the latter probably due 

 to a lateral distortion. 



Occurrence. — Ventral and dorsal valves in grey siliceous shale 

 from stone heap E. of Mount William railway station. Collected 

 l)y Prof. E. W. Skeats, D.Sc. Dorsal valve in black Lower Ordo- 

 vician shale, Lancefield. (Coll. by Dr. T. S. Hall, M.A. } pres. 

 4.3.08). 



Relationships. — The above species belongs to the group of 

 depressed Acrotretce, of which Acrofreta sagitfalis, Davidson sp.l 

 forms a central type. In that species the valves are not so laterally 

 widened as in the Victorian, and in this respect there is a nearer 

 approach to the present species in Acrofrefa helti, Davidson sp.2, 

 in which again the laterally ovoid outline is not so pronounced as 

 in A. antipodum. In the surface ornament A. sagittalis. by its 

 concentric laminae, shows closer affinity with A. aiitipodnm, but 

 it is not so strong, nor even inclined to spinosity as in the Vic- 

 torian species. A. sagittalis is a Menevian fossil in Wales, and A. 

 belli occurs in the Lower Tremadoc. 



Another related form is Aerotreta hellatula, Walcott,^^ a Middle 

 Cambrian species from Millard County, Utah. ' This is a depressed 

 form, having a more circular outline, and with the aj^ex close to 

 the posterior margin ; whilst the surface of the valve is moderately 

 smooth or finely concentric. 



1 Oholella sagittalig, Salter MS. Rep. Brit. Assoc, 186.'., p. :^S.'i. 

 Dincina labiuaa. Idem, iliid (name only). 



Obolella mgittalis, Davidson. Geol. Ma<:., vol. v., 1868, p. 309, pi. xv., fijrs. 17-24. 

 LinnarsKonia mijittalis, Dav. sp. Hall and Clarke, I'al. N. York, vol. v iii., pt. i., 1892, 

 p. 108. 



2 Oholella helti, Davidson. Geol. Mag., vol. v., 1868, p. 310, pi. xv., figs. t:.--'. 



3 Smithsonian Misc. Coll., vol. liii., 1908, p. 93, pi. i.\. fijfs. 4, 4a, b. 



