BRITISH SILURIAN BRACHIOPODA. 125 



taken for a true exposition of its interior, instead of his fig. 1 ; and, so far as the latter 

 figure is concerned, it seems to have been based, as he himself suggests it might have 

 been, upon some accidental displacement of the loop. Prof. Hall states that he based his 

 figures of Z. modesta upon preparations by Dr. C. Rominger, of Ann Arbor, Michigan. 



Zygospira Headi, Billings, sp. 



Athykis Headi, Billincjs. Geol. of Canada ; Palaeozoic Fossils, vol. i, p. 147, fig. 125, 



1865 (var. borealis, Anticostiensis, and Anglica 



excluded). 



Zygospira Headi, Hall. Notes on some new or imperfectlj' known forms among the 



Brachiopoda, extracted from ' Twenty-seventh Report State 



Cabinet,' pi. 13, figs. 21—25, 1872. 



Zygospira Headi {Billings), Miller. The American Palaeozoic Fossils : A Catalogue 



of the Genera and the Species, &c., p. 140 

 (varieties excluded), 1877. 



Mr. Billings describes his species as " Broad, oval, or sub-pentagonal ; both valves 

 convex ; sides and front occasionally somewhat straight. Ventral valve [our dorsal] 

 rather strongly convex, most elevated about the middle or a little above ; beak closely 

 incurved, in contact with the umbo of the dorsal [our ventral] valve ; umbo somewhat 

 carinated. An obscure mesial sinus, which is usually so slightly impressed as to 

 constitute only a flattening of the shell, extends from the front margin to within one 

 third of the length from the beak, where it becomes obsolete. On each side of the sinus 

 the shell descends with a somewhat flat slope to the sides. Dorsal [our ventral] valve not 

 so convex as the ventral [our dorsal], often with an obscure median sinus. Surface with 

 fine, rounded, radiating ridges, closely crowded together, of a nearly uniform size, from 

 eight to ten in the width of two lines. 



"Length 10 lines; width a little less than the length." 



Locality and Formation. — " On the south shore of the St. Lawrence, opposite Three 

 Rivers, Hudson-River formation." 



Ohs. — In PI. XXII, figs. 1 — 7, of my 'Silurian Monograph,' 186G, I figured some 

 English and Irish specimens as a variety of Billings' species ; since then I have had 

 reason to believe that we have not hitherto discovered in our Silurian rocks the true A. 

 Headi of Billings, and that the British shell I referred to that species is specifically 

 distinct from Billings' type, and referable to his so-termed variety Anticostiensis (see p. 

 127). 



In 1866 Mr. Billings informed me that the spirals in A. Headi are arranged as in 

 Atrypa. 



