322 



Vanuaoemia, and is now included in the family Cyrtodontidse, Ulrich, of 

 the order Prionodesmacea, Dall. The types of M. tener, which were col- 

 lected by Mr. J. Richardson and Dr. R. Bell in 1857, at Blue Point, on 

 Lake St. John, are still in the Museum of this Survey. 



" A second species of this genus, from the Trenton shales of Minne- 

 sota, was described by Mr. Ulrich in 1892, under the name M. ruyosa, 

 in the Nineteenth Annual Report of the Geological and Natural History 

 Survey of Minnesota. And, in his report on the Lower Silurian Lam- 

 ellibranchiata of Minnesota, published in 1897, in volume three, part two, 

 of the final Report on the Geology of Minnesota, Mr. Ulrich expresses 

 the opinion that the Modiolopsis recta of Hall, from the Niagara lime- 

 stone of Wisconsin and Illinois, is also a Matheria. 



" In the Museum of the Geological Survey there area few specimens of 

 a fourth and previously undeacribed as well as unfigured species of this 

 genus, from the Trenton limestone of Ottawa, collected many years ago 

 by E. Billings and labelled by him with the manuscript name Matheria 

 brevis. This species may now be defined and characterized as follows. 



Matheria brevis. 



25. 



o. 



esa. 



Matheria brevis. Fig. 25. Side view of the most perfect specimen collected, 

 in outline, and showing the marginal contour of the right valve. Fig. 

 25 a. The same specimen, as seen from above, to show the amount of 

 convexity of the closed valves. Both of these figures are of the natural 

 size. 



" Shell small, inflated and regularly convex, but not quite as wide as 

 high, suboval or obloug subquadrate, about one-third longer than high and 

 very inequilateral. Anterior side very short, narrow and consisting of a 

 small rounded lobe below the beaks, on each side ; posterior side longer, 

 and a little wider, in the direction of its height ; posterior end vertically 

 subtruncate at its midheight, rounding abruptly into the cardinal margin 

 above and into the ventral margin below. Ventral margin gently convex 

 but curving upward more abruptly and rapidly at the posterior than at 

 the anterior end ; superior border almost straight and nearly horizontal ; 

 umbones depressed, anterior, very nearly but not quite terminal ; beaks 

 incurved. 



