.9181 



The Ottawa Naturalist 



17 



6. Amnicola porata Say. 



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



a. Marl form, 



MacKay Lake__ 4.2 3.5 2.23 1.75 



b. Living form, 



MacKay Lake__ 4.2 3.62 2.1 1.73 



c. Chicago form.__ 5.00 4.25 2.8 2.08 

 These figures show that a. and b. are practically 



identical in measurements. 



This species has not changed at all in the area 

 under discussion but neither its marl environment 

 nor its present one in MacKay Lake have allowed it 

 to attain its maximum growth. This is seen by com- 

 paring them with normal forms from the Chicago 

 area which are much larger as shown by the figures 

 above. 



7. Limnaea galbana Say was first described as a 

 fossil and has since been found living. It was not 

 found living in MacKay Lake. The specimens from 

 the marl beds average a little larger than Say's type. 

 This species is such a persistent member of the marl 

 bed faunas throughout the Northeastern United 

 States and Canada that it seems especially fitted for 

 such a habitat. The living species prefers clear 

 water more or less in movement, which fact prob- 

 ably excludes it from the lake at present. 



8. Pisidium abditus Haldeman. 



Height Width Length 



a. Marl form 2.25 1.96 2.62 



b. Present form 3.25 2.83 4.03 



c. Chicago form Measurements not available. 



Ratio b:a 1.44 1.44 1.54 



p. abdilum is the only pelecypod found common 

 to the fossil and present faunas of MacKay lake. 

 The figures above show the present form to be 

 much larger than the fossil shell. 



It is interesting to consider for a moment the 

 pelecypod fauna. Pisidium abditum, the only re- 

 presentative in the marl, is uniformly small, and, 

 though fairly abundant, is not comparable in num- 

 bers at all with the gastropods. In the present lake 

 fauna Lampsilis radialus and L. lueolis are found 

 but in small numbers, and not far from the outlet. 

 These members of the Unionidae cannot thrive ap- 

 parently in stagnant water. On the other hand, the 

 members of the Sphaeriidae, represented by 

 Sphaerium simile and the species of Pisidium are to 

 be found in fairly large numbers in these waters. 



Depauperation. 

 An examination of the data for the species dis- 

 cussed above shows all the fossil forms with the 

 exception of Amnicola porata and Planorbis 

 bicarinatus to be smaller than their existing descend- 

 ants in the lake of to-day. Even these exceptions 

 are smaller than normal. The tables show that ex- 



clusive of ostracods only eight species are common 

 in the marl beds while sixteen are found in the pre- 

 sent lake and this number would be exceeded if 

 an exhaustive search were made. What caused the 

 depauperation of the marl bed fauna? As noted 

 above it seems probable that the bottom environment 

 had a great deal to do in this connection and that 

 the marly bottom was very unfavourable to most of 

 the species. No marl is being deposited in the lake 

 at the present time. But in the shallow bay to the 

 east the bottom is composed of this material. This 

 represents either an old marl bed in situ below water 

 level or the accumulated wash from higher beds. 

 The water is seldom more than four feet in depth in 

 this bay. A very small amount of muddy sediment 

 overlies this marl and is covered by a scanty aquatic 

 vegetation consisting mainly of algae. The mollus- 

 can life in this area is scanty, few living shells were 

 obtained, and these were mostly the ubiquitous 

 species Valvaia Iricarinaia and Amnicola porata. 

 The latter was mostly found attached to the sub- 

 merged plant stems and comparatively few were 

 found on the bottom itself. Pisidium can live 

 attached to algae and other aquatic plants, and thus 

 remain somewhat away from the influence of the 

 marl. The heavy shelled forms like the Unionidae, 

 however, must live directly on the bottom. The 

 marl acts unfavourably on such species probably 

 by clogging their gills. It is not surprismg therefore, 

 that these forms are entirely wanting in the marl 

 fauna. 



In contrast, on the western side of the lake there 

 are two different types of bottom, one composed of 

 soft mud with an abundant plant growth, while 

 the other is composed of rock covered with debris 

 both organic and inorganic. The first named area 

 provided every species obtained in the lake in great 

 abundance except Campeloma decisum, Limnaea 

 stagnalis, and Planorbis trivolvis. The two latter 

 species were found here also but were attached to 

 submerged objects and not on the bottom. The 

 rock covered with debris had also a considerable 

 fauna much greater than that of the marl beds in 

 the eastern bay so it seems that here at any rate the 

 marl bottom is perjudicial to a flourishing molluscan 

 fauna. The marl bottom is not conducive to growth 

 of many water plants. Certain algae and other low 

 forms are the most common. Such species as 

 Amnicola, small forms of Planorbis and Valvaia 

 can attach themselves to these algae but prefer lily 

 stems and pads and other plants with vigorous stem 

 and leaf growth. 



As mentioned above the marl matrix is not com- 

 posed of shell fragments but of a fine impalpable 

 powder of calcium carbonate. Two theories have 

 been propounded to account for such a type of de- 



