Report of the State GEOLOGIST. f>(i7 



by a circular foramina] aperture (?). The interior of this valve is 

 characterized by a pair of deep, diverging furrows, passing for- 

 ward from the beak or internal foramina] opening, in broad 



curves which converge toward the anterior margin bul without 

 meeting. These furrows inclose a thickened and somewhat 



elevated central area, which, in the subumbonal region, is 

 apparently free, projecting for a short distance, like a narrow, 

 triangular shelf, beneath which the foramen probably opened. 

 A faint longitudinal ridge passes from the apex of the shelf to 

 the anterior margin. 



Shell-substance tenuous, apparently corneous. External surface 

 covered with more or less prominent, sometimes lamellose con- 

 centric growth-lines, crossed by fine, gently curved, radiating stria? 

 which are usually more prominent when the concentric lines are 

 exfoliated. 



Type. Discmopsis Gulielmi, Matthew. (Cambrian.) 



A single species from the St. John group of Xew Brunswick. 



Linnarssonia, Walcott. 1885. 

 (Plate 3, Figs. 22, 23.) 



Shells transversely ovate or subcircular ; valves convex. Pedi- 

 cle-valve with somewhat elevated submarginal apex, perforated 

 by a circular foramen : without cardinal area or thickened car- 

 dinal margin. On the interior there is a conspicuous mammiform 

 tubercle surrounding the opening of the foramen, and two small 

 posterior submarginal tubercles. 



In the brachial valve are similar tubercles, though less con- 

 spicuously developed ; between them lies an axial ridge which is 

 broadened over the central portion of the shell ; at the sides are 

 shorter, divergent ridges. 



Shell substance calcareo-corneous ; lamellar. Surface smooth 

 or with concentric growth-lines. 



Type, Linnarssonia transversa, Hartt (sp.). 



Distribution. Cambrian — (0, Lower Silurian (New York, New 

 Brunswick. Canada, Sweden, Wales). 



Mesotreta, Kutorga. 1848. 

 (Plate 4, Fig. 21.) 

 Shell patelliform, depressed-conical ; apex central, perforated 

 by a foramen ; no cardinal area or flattening (3). Surface orna- 



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