16 



The Ottawa Naturalist 



[April 



1 . Planorbis companulatus Say 



H. W. A.H. A.W.* 



a. Marl from 



MacKay Lake__ 5.18 9.2 4.56 3.69 



b. Living form, 



MacKay Lake__ 6.27 11.4 5.5 4.56 



c. Form in 



Chicago area__. 6.63 12.75 5.19 4.69 



Ratio bra 1.211 1.245 1.226 1.238 



or an average ratio of 1 .23. 



In the case of Planorbis companulatus we have 

 thus conclusive evidence that the present form is 

 considerably larger (23%) and that the environ- 

 ment of the fossil species was not very suitable to it. 

 But apart from the size the species has not changed 

 at all. The ratios of the four standard measure- 

 ments are nearly equal showing that the proportions 

 of the shell have remained practically the same. 

 The shell ornamentation remains unchanged also. 



2. Planorbis bicarinatus Say 



H. W. A.H. A.W. 



a. Marl form, 



MacKay Lake__ 4.94 8.75 4.15 3.5 



b. Living form, 



MacKay Lake__ 4.75 7.83 4.17 2.917 



c. Chicago form___ 5.75 10.63 5.13 3.88 



d. Form from Presqu'ile 



Bay. L. Ontario. 6.08 11.33 5.25 4.16 

 Ratio c:a 1.164 1.215 1.231 1.09 



Analysis of the above figures shows that in this 

 case the fossil specimens are slightly larger than the 

 living forms, a conclusion not in harmony with the 

 previous result and, as we shall see, also differing 

 from that reached for the majority of the species. 

 Apparently the quiet water of Mackay lake is not 

 well suited to Planorbis bicarinatus. The two 

 broadly funnel-shaped depressions in both sides of 

 the shell expose a very large area to erosion by 

 carbon dioxide, which the water contains in con- 

 siderable amount. Such erosion is a constant drain 

 on the vitality of the animal, as the lime of the shell 

 must be constantly renewed. We have also abun- 

 dant evidence that the marl forms themselves were 

 not well adapted to their environment. They show 

 an extreme variation in the shell: the aperture 

 varies from sub-trigonal to sub-ovate ; many speci- 

 mens show traces of former apertures, as evinced by 

 transverse thickening of the shell at one or more 

 places in the body whorl accompanied by a change 

 in direction in the latter; fully fifty per cent, of the 

 specimens examined show distinct minute revolving 

 lines occurring irregularly over the shell ; the shell 



*H. height of shell in millimeter.s. 

 W. width. 



A.H. aperture height. 

 A.W. aperture width. 



is also thickened unevenly about the aperture. 

 Specimens from Presqu'ile bay. Lake Ontario, do 

 not show any such irregularities. That the slight 

 diminution in size of the specimens found in the lake 

 as compared with those from the marl is local and 

 is not a constant feature, is indicated by the 

 measurements from the Lake Ontario and Chicago 

 specimens. That the marly bottom of the ancient 

 lake was not very suitable to this form, is evident 

 from the presence of so many abnormalities in the 

 individuals, but it was a little more suitable than the 

 present lake. 



3. Plenorbis parvus Say. The fossil form is 

 variable in size. It is impossible to give an exact 

 series of measurements but the average is lower than 

 those of the living forms to-day. This species is 

 found in considerable numbers in the marl bed but 

 IS not nearly so abundant as Valvata tricarinata and 

 Amnicola porata. 



4. Physa heterostropha Say. 



H. W. A.H. A.W. 



a. Marl form, 



MacKay Lake_ J 1.38 7.38 8.69 4.08 



b. Living form, 



MacKay Lake__ 12.88 8.13 9.75 4.61 



c. Chicago form __ 13.50 8.67 10.17 4.33 

 Ratiob:a 1.132 1.102 1.122 1.111 



In the case of this species the living form is some- 

 what larger than the fossils. The ratio agree closely. 

 Ph\)sa heterostropha shows with the other species 

 the adverse influence of the marly bottom. The 

 fossil form retains some of the original coloring 

 matter in a red band inside the callus at the aper- 

 ture. 



5. Valvata tricarinata Say. 



H. W. A.H. A.W. 



a. Marl form, 



MacKay Lake__ 2.7 3.9 1.8 1.65 



b. Living form, 



MacKay Lake__ 4.41 5.47 2.67 2.28 



c. Chicago form___ 4.00 4.00 2.00 2.00 

 One specimen only. 



Ratio b:a 1.65 1.40 1.48 1.39 



Of all the species discussed in this paper Valvata 

 tricarinata shows the greatest difference in size be- 

 tween the present and fossil specimens. The linear 

 measurements show that the bulk of the living animal 

 is more than twice the size of the fossil form. This 

 species occurs in great abundance in the marl beds 

 and is uniformly small. The marl specimens might 

 be considered a dwarf variety of the species which 

 adapted itself to an unsuitable bottom environment. 

 Many of the shells are slightly green in colour. 



