NO. 2297. BRACTIIOPOD GENUS PLATYSTROPHTA—McEWAN. 393 



culmination is followed in the Upper Whitewater by loss of vitality, 

 as is indicated in the excessive deposit of lime, narrowing of the shell, 

 and loss of plications next to the cardinal angles. 



The high-fold subgroup passes through an accelerated low-fold 

 development, and in the late neanic stage develops a high compressed 

 fold on which the plications are weak. It has no representatives in 

 the Trenton. There are occasional individuals which have a some- 

 what higher fold than others, but this seems to represent Only an 

 attempt at the realization of a latent tendency which was never 

 fully expressed. 



PlatystropJiia fauciplicata is a Platystropliia juvenis, new species, 

 which has lost two of the lateral plications on the lateral slopes. 

 Variants from PlatystropJiia pauciplicata developed a high fold and 

 merged into PlatystropJiia profundosulcata by all degrees of variants. 



From some of these variants PlatystropJiia crassa developed. The 

 most abundant mutation of this species has a globose form and high 

 compressed fold and deep sinus, on which the plications are weak. 

 In the mutation which James took as his type the lateral plications 

 of the fold and sinus have disappeared or exist as rudimentary struc- 

 tures ne:^t to the beak. Another mutation has only four or five pli- 

 cations on the lateral slopes. These mutations indicate that this 

 species was on the decline, having reached the highest point of 

 specialization in the high fold mutation. 



PlatystropJiia laticosta is a transverse species with a high com- 

 pressed fold on which the plications are weak. The young of this 

 species resemble PlatystropJiia profundosulcata but the fold is not so 

 high nor the smus so deep. Index curves and the physiognomy of 

 the shell indicate that PlatystropJiia laticosta developed from one of 

 the high fold variants of PlatystropJiia pauciplicata} 



PlatystropJiia unicostata developed from PlatystropJiia laticosta by 

 all degrees of variants. It marks the culmination of a development 

 toward a high compressed fold on which the lateral plications have 

 disappeared or exist as rudimentary structures next to the beak. 

 But with this high specialization came extinction, as loss of vitality 

 is indicated by the narrowing of the shell, increase in gibbosity, and 

 loss of plications next to the cardinal angles. The variety crassi- 

 formis marks extreme racial gerontism, as there are only four or 

 five plications on the lateral slopes and the form has become so nar- 

 row and gibbous that the thickness equals or is even greater than the 

 width and height. 



The physiognomy of PlatystropJiia cypJia of the Arnheim formation 

 suggests its development from PlatystropJiia unicostata, but this is 

 not likely, as the latter species is taking on declining characteristics. 



1 Professor Cumings has demonstrated the origin of Platystrophia crassa and Platystrophia laticosta from 

 Platystrophia pauciplicata. Amer. Joum. Scl., ser. 4, vol. 15, p. 23. 



