ORTHIDES OP THE HAMILTON GROUP. S9 



Ventral valve convex near the beak, becoming broadly flattened below 

 the middle, and sometimes ( perhaps from pressure) a little concave 

 towards the margin ; front without (or Avith a scarcely perceptible) si- 

 nuosity ; area of moderate width, a little concave, and the beak slightly 

 incurved : the foramen is large, and nearly filled by the cardinal process 

 of the opposite valve. 

 Surface marked by fine bifurcating subangular striae, which are marked 

 by poriform openings upon the surface, and with the intermediate space 

 finely punctate. There are a few lamellose lines of growth in the midde 

 of the shell, and a larger number on the margin. 



This species is one of the type of 0. oblata and 0. vanuxemi, differing from the 

 latter as 0. discus does from the former. Although its measurements differ but 

 little from 0. vanuxemi, its expression is that of a more nearly circular shell, and . 

 it has maintained this form from its earlier stages of growth. Owing to this 

 feature, the striae along the hinge-line curve a little more abrupt upward. 



This species differs more distinctly from 0. vanuxemi than either 0. leucosia or 

 O. penelope; though it is not easy to point out these distinctions, or illustrate 

 the same in figures. It is probably identical with the species described from the 

 Corniferous limestone on page 39, and figured on Plate v, f. 5 ; but that one is 

 a little distorted by pressure. 



Geological formation and locality. In the Hamilton group at Moscow, and atj 

 Eighteen-mile creek on Lake Erie. 



In species so similar as some of the Orthides of this and other groups, 

 it is difficult to point out characters by which they may be distinguished. 

 The measurements of proportions, although often valuable, will be of 

 little use if too far relied upon ; while the degree of fineness or coarseness 

 of the strioB, or the number of striae within a certain space, will serve to 

 distinguish species when there is considerable difference in the strength 

 of these features, but I do not consider that this can be relied upon in 

 species approaching each other. 



I have given below some measurements of proportions of parts, and 

 of number of striae, of authentic specimens of several species, for the 

 purpose of showing the degree of variation observed. I should mention, 



