12G SUPPLEMENT TO THE 



In 1872 Prof. Hall published some notes on Z. Ileadi in the ' Twenty-seventh Report 

 of the State Cabinet.' In these notes Prof. Hall gives the 

 accompanying outline figure, by Mr. Whitfield, " showing the 

 characters of the spire as actually seen in a specimen partially- 

 filled with crystalline matter, and cut to show the internal 

 arrangements of one half of the spire and loop." The direction 

 of the spirals in this figure is the same as in the genus Zygo- 

 spira. The loop, however, differs from that of Z. modesta as 

 much as the loop of the latter differs from that of Atrypa. 

 According to Prof. Hall's figure of Z. Headi its loop is attached to the primary lamellae 

 higher up than the loop of Z. modesta, and, in this respect, the loop of Z. Headi is shown 

 to be similar to that of Atrypa. Prof. Hall's figure indicates, however, that the loop of 

 Z. Headi differs from that of Atrypa in the breadth and depth of the V-shaped process, 

 which in Z. Headi is in close apposition to the posterior border of the spirals. 



Zygospira erratica, Hall, sp. 



Orthis? ekratica, Hall. Pal. New York, vol. i, p. 288, pi. 79, fig. 5, 1847. 



Shell marginally broadly rounded, or slightly broader than long, greatest breadth 

 about the middle, hinge-line rather long and nearly straight. Ventral valve convex 

 longitudinally, somewhat carinated or flattened along the middle ; beak incurved, beak- 

 ridges sharply defined, and leaving between them and the hinge-line a narrow flattened 

 space or false area. Dorsal valve convex at the sides, depressed longitudinally along the 

 middle, and presenting a concave line in front. Surface marked by fine, uniform, 

 radiating, raised strise, increasing here and there in number by the interpolations of 

 shorter and smaller ribs. 



Length 7, width 1\, depth 4 lines. 



In 1847 Prof. Hall figured and described this fossil as an Orthis, but with a note of 

 interrogation. Dr. G. J. Hinde having sent a number of specimens to the Rev. N. Glass, 

 the latter was enabled to examine its internal characters, and these he found to agree with 

 those already described for Z. Headi. It is therefore a spiral-bearing species, and not an 

 Orthis. Z. erratica also bears some resemblance to Z. Headi in its external form, 

 especially as in both species there is a somewhat deep sinus in the dorsal valve. 



We have not found this fossil in Great Britain. Prof. Hall states it to be 

 abundant in the Hudson-River group, near Washingtonville, Oswego County, America. 

 Dr. G. J, Hinde informs me that he got the fossil in the Cincinnati group, from Weston, 

 near Toronto. It remains, however, to be determined whether Z. erratica can be speci- 

 fically distinguished from Z. Headi. 



