592 GEOLOGY OF OHIO. 



to Atrypa marginalis'. It has developed, however, new characters of 

 sufficient distinctness 'and constancy to be. regarded as a new species, 

 and it is described here as lata-corrugaia, a form regarded as related to 

 Atrypa marginalis genetically. The median fold of the brachial valve 

 and its lateral denning depressions are /visible only in young individuals 

 and are retained only near the beak, within 6 to 8 mm. of the same, in 

 ddult specimens; but since at this stage the median fold never rises 

 above the general surface of the shell, the appearance is rather that of 

 two diverging grooves, disappearing at the first strong concentric 

 wrinkle, or at least showing but faint traces anterior to this point. The 

 median sinus of the brachial valve has left yet fewer traces^ Its posi- 

 tion is indicated by the strong convexity of the shell along a median line 

 just anterior to the beak, but the traces of the sinus are confined to the 

 existence of two radiating plications which occupy the position of the 

 sides of the sinus, and which are raised either indistinctly, or slightly, or 

 more or less distinctly, above the general surface of the shell, but the de- 

 pression which ought to lie between these lines does not exist. As a 

 rule, no traces of either the median sinus nor the fold appear over the 

 middle or the anterior regions of the shell, but in one specimen a broad 

 low elevation, with its corresponding depression in the other valve, indi- 

 cates a reversal of this specimen to its former habits of growth in its 

 old age. The surface of both valves is marked by radiating plications, 

 and by a series of concentric wrinkles, which are so strong and so distant 

 from each other as to form the most conspicuous feature of this variety, 

 and to give name to the same. The partial retention of the characters 

 of Atrypa marginalis in the young of A. lata-corrugata is an interesting 

 illustration of similar facts in higher orders of animal life. 



Rhynchonella scobina, Meek. 



Rhynchonella scobina is a very frequent shell in many localities in 

 Ohio. Corresponding to its general distribution is a wide range of 

 variations in form. The typical specim«ns present a rather rotund out- 

 line, when mature; the.postero-lateral sides of the pedicle valve form a 

 wide angle, often 90 degrees or more, and the straight contour of the 

 shell along this region is too. short to make the beak break in very much 

 upon the general rotundity of outline. Both valves are very convex. 

 The adult pedicle valve has a deep sinus, and the brachial valve a high 

 well defined median fold, with steep sides. Three plications are found 

 in the sinus of the pedicle valve, and four on the median fold of the brachial 

 valve; in each case the two exterior plications are much less elevated 

 and narrower. The line of junction of the valves is very sinuous at 

 this fold or sinus. A variation of this type has an equally strong con- 

 vexity of shell, but the width is so great as to exceed the length. The 

 young of both of these forms show a rather rotund outline, and a consider- 

 able convexity of shell, even when only 7 mm. long.- In a third variation 



