BRITISH SILURIAN BRACHIOPODA. 127 



18. Ztgospira anticostiensis, Billings, sp. Dav., Sil. Mon., PI. XXII, figs. 1 — 8. 



Athyris (?) Headi, t'fitr. anticostiensis, and var. borealis, Billings. Geol. Survey of 

 Canada ; Pal. Fossils, vol. i, p. 147, figs. 126 and 

 127, 18C5 (not ^. Headi). 



Ateypa Headii, var. Anglica, Dav. ? Sil. Mon., pi. xxii, figs. 1 — 8, 1865. 



Shell circular or longitudinally oval, broadest about the middle ; dorsal valve 

 uniformly and strongly convex, especially at the umbo ; no fold. Ventral slightly deeper 

 than the dorsal valve, with a median depression or longitudinal groove commencing 

 at the beak and extending to the front, dividing the valve into tvi^o equal halves ; beak 

 strongly incurved ; lateral margins nearly straight, front line raised into a rounded wave 

 or convex curve. Surface of valves covered with numerous radiating, fine, rounded, 

 raised striae or small riblets, closely crowded together, and nearly uniform in size. Two 

 specimens measured respectively — 



Length 7, width 6, depth ^\ lines. 

 » 6, „ 6, „ 5 „ 



Obs. — In vol. i, p. 147, of the ' Geological Survey of Canada,' "Palaeozoic Fossils," 

 Mr. Billings figures this species as a variety of his Athyris {?) Headi, adding that it is 

 " smaller than the typical form, more nearly circular, and with a more tumid umbo." 

 He also describes and figures as a variety of Headi, and by the name of borealis, 

 another shell, which he states to " differ from the typical form in being more elongated 

 oval, and in having a more tumid umbo." Upon the examination of a large number of 

 specimens of Anticostiensis and borealis Mr. Glass and myself have arrived at the opinion 

 that they are only variations in shape of the same species, but specifically distinct from 

 the Zygospira {Athyris ?) Headi of Billings. 



In Zygospira Headi the shell is broadest posteriorly ; in Zygospira anticostiensis at the 

 middle of its length. Headi in medium-sized specimens is broader than long, big 

 specimens as broad as long ; in Anticostiensis the specimens are regular, oval or circular ; 

 but the most marked external characters consist in Headi having in the dorsal valve a 

 somewhat deep longitudinal depression or sinus, while, on the contrary, Anticostiensis has 

 the sinus in the ventral valve. This afiects in both the form of the frontal line, which is 

 concave in the one {Headi), convex in the other {Anticostiensis). 



Dr. Hinde having kindly forwarded to Mr. Glass a number of specimens of 

 Z. Anticostiensis, the latter was able to develop the spirals and their attachments 

 in a most satisfactory manner. He found that the spirals were placed in the shell 

 similarly to those of the other species of Zygospira; and he found that the loop 

 resembled in shape that which is figured by Hall for Z. Headi, with this marked 



18 



