168 PALEONTOLOGY OF NEW YORK. 



(Lorraine) shales of central New York, that is, the pedicle-cavity is deep, and in 

 front of it lies a more deeply excavated, short, sharply defined and longitudinally 

 striated impression. In the brachial valve is a broad anterior and a narrow, elon- 

 gate posterior pair of scars. The spirals are of essentially the same character as 

 in Z. modesta, though the form of the cones is such that their apices converge 

 toward the median line in a plane just below the surface of the brachial valve. 

 The loop, however, diflfers ; in the first place, it is 

 persistently posterior in its position, originating as 

 in Atrypa, the lateral lamellae bending downward 

 toward the bottom of the brachial valve and 

 directed forward in lines which are parallel for a 



*^ Fig. 151. 



short distance. Thence they bend inward and P'lP'^tionot catazyga aeadt mmngs; 



•' snowing the form of the spiral cones 



upward, meeting in a sharp angle in the space just ^^^f^^"'"'' "" "'*'"*'* '"'''"" "'" ^;;",'='""' 

 behind the apices of the spirals. 



These external and internal peculiarities afibrd a sufficient basis for the sep- 

 aration of shells of this type from Z. modesta and its allies. Mr. Billings de- 

 scribed two varieties of A. Headi, viz., A. borealis, from Lake St. John and the 

 Saguenay River ; and A. Anticostiensis, from Anticosti, both from the Hudson River 

 formation. The former " differs from the typical form in being more elongate- 

 oval and in having a more tumid umbo" {op. cit, p. 147). This elongate variety 

 prevails in the Hudson River fauna of Ohio, though in association with shells 

 agreeing with the typical A. Headi. The variety Aniicostiensis retains more of 

 the contour of a Zygospira, the pedicle-valve being more prominently keeled, 

 the convexity more unequal, and, furthermore, there is a broad sinus on the 

 brachial valve, while there is no trace of one on the other valve. This fossil is 

 like Z. erratica, both in contour and in the fine striation of the exterior. If 

 there is a specific difference in the two forms it is extremely slight, too slight, 

 indeed, with our present knowledge, to indicate either in words or illustration. 

 These forms are interesting as being intermediary between the typical Zygospira 

 and the representatives of the proposed subgenus Catazyga.* 



*0ur specimens ot AthyrU Headi and its variety A. Antict>stieiisis are from the original localities, and 

 frt)m the investigation of these we ai-e compelled to disagree with Mr. Davidson's determinations given 

 upon pages 126-128 of his Supplement to the British Bi-achiopoda. On page 127 he states that 



