22 ( kt m Ings — Morphogenesis of Platystrophia. 



No8. 1 7-3 1 inclusive, fig. 8, represents the contours of a series 

 of specimens from the Trenton of Kentucky (Colby Co.). 

 Here the index varies from 1*24 to 1*51; the average being 

 1*4 and the largest class 1*4. The cardinal angle varies 

 from 90° to 112° (in a specimen from Harrodsburgh, Mercer 

 Co., Ivy., the cardinal angle is 120°). The number of plica- 

 tions varies from 17 to 27 ; the number in the sinus from 3 to 5. 

 No. 33, fig. 8, is a composite of Nos. 17-31 inclusive. This 

 8 normal shell of the Kentucky group 



has an index of 1*40 and a cardinal 

 angle of 95°. JSTo. 34, fig. 8, is a com- 

 posite of JSTos. 32 and 33, and has an 

 index of 1*41 and an angle of 93°. The 

 great similarity of the New York and 

 Kentucky forms is apparent. 



Very few specimens from the Tren- 

 ton of Minnesota have been studied, but 

 Fig. 8 bis. Diagram show- these show no departure from the above 



ing method of obtaining type> ^he gpecimens f rom t ^ e Q a l e na 

 cardinal angle bac. *v \ T . , c -, .■,.-, ,-, 



oi Minnesota are however more like the 

 Cincinnati group lynx. 



In brief, the Trenton shell is rather transverse, slight^ shorter 

 on the hinge than farther forward ; valves about equally con- 

 vex, never extremely gibbous ; plications about 22, of which 

 three or four are in the sinus, four or five on the fold. The 

 sinus is never as profound as in the varieties costata, laticosta 

 and acutilirata. 



The Trenton forms just described are from the middle and 

 upper part of the series. Some specimens from Montreal, 

 Canada, loaned me by Mr. Chas. Schuchert, are of a somewhat 

 different and more primitive type. These latter (fig. 9) have 



<£>. 



d 



Fig. 9. Platystrophia lynx, small pauciplicate form from the lower Tren- 

 ton at Montreal, Canada, a, ventral ; b, anterior ; c, profile ; d, posterior 

 views. Schuchert collection. 



but 4-7 plications on the slopes ; the shell is narrowest at the 

 hinge-line and considerably less transverse than the Trenton 

 Falls type (the index is 1*23 in the specimen figured, which is 

 about an average individual). The second and third plications 

 of the sinus arise about 4 mm from the beak. These shells strongly 

 suggest the small pauciplicate form found at the base of the 

 Lorraine at Cincinnati, Ohio.* As to the exact age of the beds 

 from which these specimens came it is impossible to say. In 

 *See Winchell and Schuchert, Geol. Minn., iii, 1893, p. 456. 



