50 G. F. MATTHEW UN THE 



This species has a low umbo for an Orthisiua, and in its form recalls the genus Stropho- 

 mena, as also do the concentric undulations that are found on the back of the ventral valve. 

 The dorsal valve, however, is convex, and the area of this and the ventral valve too high 

 for a Strophomena. It does not appear to agree with any described species of Orthisina. 

 It resembles O. orientaiis, White, somewhat in form, but is not so long nor so wide at the 

 hinge. From O. pepina, Hall, it diifers in its lower umbo and area, shorter hinge and 

 smoother surface. It approaches more closely to the form from the Potsdam Sandstone of 

 the West, figured by Prof. Hall but not named, except as a " Strophomena or Stropho- 

 donta." ' 



AaNOSTUS. 



Agnosttjs bisectus, n. sp. (PI. XIII, figs. 2 a and b.) 



Body oval, somewhat quadrate behind. Crust smooth. 



Head shield somewhat wider than long. Marginal fold of moderate width. Cheeks 

 wide, connected in front. Grlabella of two lobes, the anterior short, rounded in front ; the 

 posterior prominent, elevated behind. Basal lobes small, sub triangular. 



Pygidium subquadrangular, roiinded behind. Marginal fold with two lateral spines. 

 Lateral lobes of nearly equal width all around. E.achis divided into two lobes of nearly 

 equal length, the anterior sloping up from the front to a tubercular point, the posterior 

 sloping rapidly hack to a rounded termination. The obsolete anterior lobe is represented 

 by two small lateral lobes, one on each side of the anterior end of the rachis. 



Size. — Length of the body, T mm. ; width,*3 mm. 



Horizon and Locality. — In the black shales of Div. 3 6 at Navy Island, St. John. 



This little species of the section Limbati is of interest as another connecting link by 

 which the earlier Limbati of the Paradoxides beds were connected with those of the Ordo- 

 vician system. A. fallax of Sweden and A. vir of Acadia are two of the earlier forms ; the 

 latter is represented in the fauna of Div. 1 dhy var. concinnvs and A. fallax by var. minor 

 near the same horizon in Sweden. But the type is wanting in the Olenus beds of Sweden, 

 and appears again with us in Acadia in this species (A. bisectus). The type reappears in 

 Sweden at a higher horizon in the species A. Sidenbfadlii, Lnrs , of the Ceratopyge limestone. 

 From this species the Acadian one differs in having only two lobes in the pygidium. 

 Had the tubercle on the anterior lobe of the rachis of the pygidium in our species extended 

 forward as a narrow elongated lobe to the front of the pygidium, our species would have 

 been almost exactly A. Sidenbladlii, both in size and ornamentation. 



In the Calciferous and Chazy limestone and shales (M and N) of northern New- 

 foundland occurs Billings' species A. Gulba, which is closely allied to A. Sidenbladlii, differ- 

 ing very little except in the absence of marginal spines, and these are very apt to be 

 overlooked. This carries the type up to the Ordovician system. 



In the Ordovician system, in the Expansus Shale, occurs Angelin's species .4. glabratus, 

 also of the Limbatus type. This differs from the Acadian species herein described, in 

 having no lobes in the glabella and having a tapering rachis to the pygidium. 



In the Oaradoc or Bala limestone of Wales a later phase of the Limbatus type of 



' " Preliminary Notice of the Fauna of the Potsdam Sandstone,'' plate vi, fig. 22. 



