10 ( kt m hujs — Morphogenesis of Platystrophia. 



however, it is necessary to state that tlie method the}' describe 

 has only been observed in an occasional specimen of Upper 

 Silurian (biplicate)* type and therefore does not apply to any 

 of <>ur Ordovician PI (dystrophias above the basal Trenton. 

 In no case have I seen a mesial sinus in both valves, at any 

 stage in development. The true method of origin of the 

 plications in American Ordovician forms was pointed out by 

 Hall in 1847. f 



It remains to note some apparent exceptions to the above 

 rule in regard to American Ordovician forms. Very rarely in 

 the Cincinnati group an individual of Platystrophia costata% 

 occurs with three full plications on the fold and two in the 



Fig. 4. A and C, specimens of a panciplieate lynx from Cincinnati, Ohio, 

 in which the third plication of the sinus is suppressed, causing the shell to 

 simulate P. dentata, shown at B. In C the lateral plication of the sinus is 

 clearly seen to be implanted ; and less clearly though just as certainly in A. 

 In P. dentata as shown in B (a specimen from Eussia) the primary plication 

 of the sinus bifurcates. A and C from the Dyer collection, Museum of 

 Comparative Zoology, Harvard University. B from collection of the U. S. 

 National Museum. 



sinus. In the Dyer collection at Harvard, which contains 

 several thousand specimens of this variety, I noticed perhaps 

 half a dozen such individuals. In the majority of these the 

 type is manifestly triplicate, since the second plication of the 

 sinus is implanted at the side of the primary and does not 

 originate by the bifurcation of the primary. The third plica- 

 tion is simply suppressed. I saw but two individuals (fig. 4, 

 A and C) in which the primary plication seemed to bifurcate, 

 and even here a close inspection of the beak shows that the 

 second plication is really implanted. 



Another apparent exception is that of the type in which 

 there is but one plication in the sinus. A careful examination 



* It is convenient to speak of forms of Platystrophia having two main 

 plications in the sinus, originating as just described the Silurian type, as 

 biplicate. Those having three characteristic plications, as in nearly all 

 American Ordovician forms, are similarly called triplicate ; and those hav- 

 ing but one plication in the sinus, uniplicate. 



f Pal. N.Y., vol. i, 1847, pp. 133-134. 



\ The American form known as dentata Meek, and crassa James, has 

 been shown to be the same as costata Pander. See this Journal, July, 1902, 

 p. 14, footnote. 



