23^ ROYAL SOCIETY OF CANADA 



together 18.8, hinge line 23.2 mm. The plications bounding the sinus 

 of the pedicle valve, toward the front are less elevated, and thus less 

 conspicuous than the second on each side, and the plications on the sides 

 are less strongl}^ developed than in the normal forms of the species. 



The number of plications visible on the pedicle valve outside the 

 sinus is 10, but they are of the size of those other specimens in the series 

 which are salient at the cardinal extremity and show 12. The specimen 

 is evidently an extreme variation of the specimens listed as Sp. con- 

 cinnvs; the difference consisting in more rapid concentration of growth 

 in central region increasing the rate of lateral forward growth, en- 

 larging the median portion and diminishing rate of lateral growth re- 

 latively along the direction of cardinal line. The cardinal margin mea- 

 sures 22.6 mm. while greatest width, about half way to front is 23.4. 

 This interpretation of the nature of tlie variations is shown also in the 

 relatively larger size of the first three plications each side the sinus than 

 ihose farther out. Tlie variety approaches S. cyclopterus but the plica- 

 lions are finei- and more numerous. 



T find no specimens in this lot which may be properly referred to 

 the species S. cyclopterus, though that species is reported in the collec- 

 tions studied by Donald, Decks and Ami. In lot 1 (252.1) there are 

 specimens which I regard as the form called 8. cycloptera, Hall, by 

 Billings, but in form and dimensions they agree more closely with 

 Hall's species Sp. tribuKs. 



24. Cyrtinca Dahnani, Hall. 



A few single specimens including both valves present the characters 

 of 'this species as seen in the Lower Helderberg rocks of the New Yopk 

 sections. 



25. Airypa reticularis Linnœus. 



Airy pa reticularis is abimdant in the fauna, agreeing in average 

 iSize, form, gibbosity, and surface lineations with the form as it is re- 

 presented in the Helderberg of Xew York and also exhibiting the range 

 of variability seen at that horizon. 



26. Meristella princcps, Hall. 



A few more or less perfect shells and showing both valves, have the 

 general form with long sloping sides, fold and sinus faintly expressed 

 and size of the smaller specimens referred by Hall to the species 

 Merlstella princeps in his original description of the Helderbergian 

 fauna. Our largest specimen is scarcely 30 mm. long. 



27. Meristella lœvis, Vanuxem. 



Some smaller ovate forms, the beak of pedicle valve short and 

 ali'ching, a slight sulcus at the front of the pedicle valve are referred 

 to Vanuxem^s species Meristella lœvis. 



