ee a ae 
PART I CHAZYAN AND RELATED BRACHIOPODS—COOPER 1005 
costellate than F. convexa. The species is probably new, but better material 
should be had before it is named. 
Genus HIRNANTIA Lamont, 1935 
Plate 145, D, figures 24-28 
Hirnantia Lamont, Trans. Geol. Soc. Glasgow, vol. 19, pt. 2, p. 313, 1935.—A. WILLIAMS, 
Quart. Journ. Geol. Soc. London, vol. 107, p. 97, 1951. 
This genus was proposed in 1935 by Lamont, but it has never been described 
or its essential characters given in detail. The allusion to its relationship to 
Fardenia is incorrect. Several specimens of the genotype in the National Col- 
lection from the Bala of Hirnant Valley, Wales, collected by Thomas Ruddy 
show all the important anatomical details except the character of the shell 
substance. 
The specimens mentioned indicate a large, subrectangular shell having fine, 
fairly even costellae. Inside the pedicle valve the dental plates are flaring and 
short, surrounding a broadly oval muscle field. The diductor scars are subtri- 
angular and moderately large; the adductor scars combine to form a wide and 
triangular field between the diductors. 
The brachial valve is characterized by flaring, strong brachiophore supporting 
plates. The cardinal process is small with a moderately long, slender shaft sur- 
mounted by a small lobate myophore. The median ridge is low and extends 
nearly to the valve middle. 
Genotype (by original designation).—Orthis sagittifera M’Coy, Ann. Nat. 
Hist., 2nd ser., vol. 8, p. 398, 1851. 
Discussion—Lamont thought that his genus was closely related to Fardenia, 
differing from that genus in “having a simple cardinal process, markedly greater 
convexity of dorsal valve, and radii of nearly equal calibre.” The entire structure 
of Hirnantia is really quite different from Fardenia and indicates relationship to 
Pionodema rather than any of the schuchertellids. The pedicle musculature is 
like that of Pionodema but differs in having a wider adductor field and in lack- 
ing the median elevation usually associated with the adductors. The brachial 
interior is almost identical with that of Pionodema. The cardinal process is 
fairly long shafted, and the shaft is surmounted by a small, lobate myophore. 
The most distinctive feature of Hirnantia, and the feature most like Pionodema, 
is the presence of brachiophore supporting plates. These are erect and flaring. 
Fulcral plates are visible on one of the specimens (23960f). 
Hirnantia differs from Pionodema in details of the ornamentation, the mus- 
culature of the pedicle valve, and the stronger development of the structures of 
the brachial valve. Nothing is known about the structure of the shell, whether 
punctate or impunctate. Because of the similarity of the structures of the brachial 
valve to those of Pionodema, Hirnantia is here placed in the same family as the 
American shell. 
Types.—Hypotypes : 23960a,b,f,g. 
