10 Guwdngs and MaucTc — Variation in the 



(equals sinus- index) ; number of plications on ventral valve ; 

 number of plications on dorsal valve ; number of plications in 

 sinus; number of plications on fold. ^ 



The width of the shell is in every case the greatest width, 

 whether this occurs at the hinge line or farther forward. It 

 was obtained by means of adjustable spring calipers. The 

 width of the sinus was obtained by spreading the points of 

 spring dividers between the anterior lateral angles of the sinus. 

 The depth of the sinus was obtained by spreading the points 

 of the dividers from one anterior lateral angle of the sinus to 

 the anterior extremity of the first adjacent furrow in the bot- 

 tom of the sinus. All measurements were read off on a milli- 

 meter scale and are correct to within 0*25°'°^. 



The number of shells used varies for the several characters 

 indicated above, on account of imperfect material. Only 

 entire shells were used for measurements ; but the number of 

 plications, especially the number in the sinus, may frequently 

 be determined with accuracy on very poorly preserved speci- 

 mens. The number of shells used in determining any one 

 character is called a group. 



Group I, Shell Index. 



Width divided by length. Number of shells used, 679. 

 E-ange of variation from 1-0 to 1'8. Modef at 1"3, with fre- 

 quency of 300'4:.:f Yariation here is moderate in amount, and 

 in the direction of greater width than length. 



Group II, Sinus Index. 



Width of sinus divided by depth of sinus. ]N"umber of shells 

 used, Q64:. Range of variation from 0*9 to 3"0. Mode at I'T 

 with a frequency of 112-2. Yariation great in amount, and in 

 the direction of a shallow sinus. 



Group III, Number of Plications on Yentral Valve. 



Number of shells used, 1173. Range of variation from 10 

 to 28. Mode at 17 with a frequency of 176*3. Yariation 

 large in amount, and in the direction of the greater number of 

 plications. 



*Prof. Shaler has given (Foss. Bracli., OMo Valley, p. 43) a series of meas- 

 urements of a limited number (20) of specimens of -Platystrophia. His series 

 includes also forms from Richmond, Indiana, and other American localities. 

 In a forthcoming paper I shall take up the discussion of material from all 

 the provinces both American and European, where Platystrophia is known 

 to occur. — E. E. C. 



f The term mode means the highest point of the curve, i. e., the class with 

 the greatest frequency. 



X For the purposes of comparison all frequencies were reduced to frequencies 

 per thousand. 



