194 C. D. Walcott — Fauna of the Upper Taconic 



bisulcata* has a similar outline, but the tubercle is unlike that 

 of A. rotundata and it is different^ situated on the valves. 



It is distinguished from A. Troyensis by its form and also the 

 elongate tubercle or ridge. 



The discovery of this species and the generic identification 

 of A. Troyensis adds another Silurian genus to the Cambrian 

 fauna and extends its range from the true Silurian down to the 

 middle Cambrian. As yet I do not know of the presence of 

 the genus in the Lower Silurian (Ordovician) rocks. 



Formation and locality. — Cambrian. Limestones, interbedded 

 in the shaly Taconic slates on M. C. Tefft's farm, about two 

 miles southeast of North Granville, Washington County, 1ST. Y. 



MlCEODISCUS CONNEXUS, U. Sp. 

 Plate I, figs. 4, 4&. 



Head semicircular, convex ; bordered by a well-defined rim 

 that is crenulated across the front and narrowed posteriorly 

 toward the glabella where it terminates; cheeks most promi- 

 nent at the postero-lateral portion, from whence they slope to 

 the deep dorsal furrow about the glabella ; the glabella and its 

 backward spinose extension form, together, a fusiform median 

 lobe, as there is no occipital furrow or ring, and the glabella 

 and the spine are continuous. The glabella approaches the 

 frontal margin more closely in some specimens than in others. 

 The surface of both the head and pygidium appears smooth 

 under a strong magnifying glass. 



A glance at the head of this species recalls Microdiscus punc- 

 tatus, M. punctatus var. Pulchellus and M. Daivsoni of the 

 Lower Cambrian. It has the frontal rim and form of M. Daw- 

 soni, but it is a smooth, not granulose species ; and the associ- 

 ated pygidium is unlike that of M. Daivsoni. To M. punctatus 

 it is related by its general form, but differs in the more coarsely 

 crenulated margin, the form of the cheeks and its smooth sur- 

 face, also in the' characters of the associated pygidium. The 

 presence of this tj^pe of the genus Microdiscus in association 

 with well-known Middle Cambrian or Taconic fossils is another 

 link between the Lower Cambrian fauna of New Brunswick and 

 the Middle Cambrian fauna. It is the first instance known to 

 me of the occurrence of a species with the long nuchal spine 

 above the Paradoxides horizon in America. 



Formation and localities.— Cambrian. Limestones, interbedded 

 in the shaly Taconic slates, on the roadside just west of Low 

 Hampton crossing of the Poultney river, one mile west of 

 North Hebron, and two miles south of North Granville, Wash- 

 ington County, N. Y. 



* Barrande; Syst. Sil. Boh., vol. i, Supplement, p. 477, 1872. 



