<or 



19181 



Recent fauna. The fauna is somewhat younger than] 

 the Pleistocene fauna on Cayuga Lake described by 

 C. J. Maury. t They occur very uniformly distri- 

 buted throughout the beds. They can be picked up 

 where the marl has weathered out, or can be pro- 

 cured by thousands if a block of unweafhered ma 

 lerial is soaked in a pail of water. Many of the 

 shells contain quantities of air and, as the block 

 disintegrates, they rise to the top and can be poured 

 off into a sieve. In this way several thousands of 

 specimens were procured for study. The original 

 coloration of the shells has disappeared and they 

 are white and opaque except in two species 

 Valvaia tricarinala, which still retains in many cases 

 a tinge of green, and Ph\)sa heierostropha. which 

 retains a red band inside the callus at the aperture of 

 the shell. The specimens are perfectly preserved for' 

 study, as the finest striae remain unabraded. 



Along with the fresh water shells were found, 

 rarely, several species of terrestrial gastropods that 

 had evidently been carried into the lake by small 

 rills or in the case of minute delicate shells, by the 

 wind, and deposited with the fresh water forms. As 

 terrestrial shells are rare in the marl and no very.., 

 definite relationships to living forms can be estab-* 

 lished in their case, they are not further discussed 

 here. 



MacKay Lake with its somewhat stagnant water 

 is not an especially good habitat for the Mollusca 

 and yet we find a considerable assemblage of forms 

 thriving it in. Many of these forms also occur 

 fossil and the following table shows the species 

 common to both. These two lists show a very 

 marked contrast in the composition of the living and 

 fossil faunas of the lake. It may be added that the 

 list of the present fauna is not complete; to make it 

 so would necessitate a larger series of dredgings 

 than it was possible to make at the time. 



Marl Fauna. Present Fauna, 



gastropoda. 

 Limnaea galbana"^ Limnaea stagnalis appressa 



Planorbis companulatus. Planorbis companulatus. 

 Planorbis bicarinatus. Planorbis bicarinatus. 

 Planorbis parvus. Planorbis parvus? 



Planorbis trivolvis. 

 Planorbis deflectus. 

 Planorbis exacuius. 

 Planorbis hirsutus. 

 Ph\fsa heierostropha. Physa heierostropha. 

 Amnicola porata. Amnicola porata. 



Pomaliopsis lustrica. 

 Valvaia tricarinala. Valvata tricarinala. 



Campeloma decisum. 



flnterglacial fauna in Cayuga Valley, .lour, of 

 Gaol. 1908, vol. XVI, pp. r,6n-567. 



*H. M. Ami lists Limnaea stagnalis and L. 

 dedidiosa from the marl beds, Vol. XII, p. 56G, Ann. 

 Rept. Geol. Surv. of Canada. 



The Ottawa Naturalis^; 



PELEC YP0D,^J^^**'5<L* S>^..V^ 



15 



f*isidium abdilum.^^ Pisidium abditum. 



Lampsilis radiatus. 



Lampsilis luteolis. 

 ostracoda** Sphaerium simile. 



K^})pris sp. 



[flSeveral other species. 



It will be seen that seven species only can be 

 used for comparison between the living and fossil 

 forms. Each of these will be taken up in detail to 

 note what differences, if any, have arisen since the 

 deposition of the marl beds. In the case of gastro- 

 pods, four sets of measurements were made in the 

 following order: (1) Height of shell, (2) Width of 

 ,shell, (3) Height of aperture, (4) Width of aper- 

 [jture. In the case of the pelecypods, length, height 

 and width were noted. Measurements were made 

 of a series of average individuals and the means of 

 l^these taken as the measurements of the species. As 

 conditions in MacKay Lake may not be favourable 

 fto the growth of a completely normal fauna, the 

 i measurements are added of the species recorded by 

 I Mr. Frank Collins Baker*** from the region about 

 1 Chicago, where the conditions are very favourable 

 to molluscan life and individuals may be expected 

 to reach a normal size. 



The method employed in determining the 

 measurements was as follows: The specimens of 

 one species were placed on a smooth surface. A 

 straight edge was then pushed through the shells 

 and a dozen or more were separated from the rest 

 and arranged in a row along the rule. This process 

 was repeated until all the specimens were arranged 

 in rows. The individuals at each end and the one 

 in the middle of each line were measured. For 

 instance, if one line contained seventeen shells, 

 numbers I, 9, and 17 were selected for measure- 

 ments. Means were calculated for each species. In 

 this way it was thought a fair average would be 

 obtained in lieu of measuring hundreds of specimens. 



A second method secured some additional data, 

 and also acted as a check on the first. The shells 

 were spread out and eight of the largest normal 

 individuals both of the living and fossil forms 

 selected and measured. It was found that the 

 measurements taken in this way were slightly greater 

 than those obtained by the first method, but that the 

 ratios of living to fossil forms agreed very closely. 

 In the tables given in this paper the results obtained 

 by the first method are used except where otherwise 

 noted. 



**None previously recorded from these beds; all 

 are probably new. 



***The Mollu.sca of the Chicago Area, Chicago 

 Acad. Sciences, Bull. No. 3, Nat. Hist. Survey. 



