this type of pupil, we find that many of them are essentially nocturnal in habits-the gecko, the alligator, the cat-animals in which the rod functions are predominant over those of the cones. At night or in dim light the pupil of the cat is wide open and round. Furthermore, in the daytime, when the pupil is a vertical slit or oval, cats hunt and catch a great deal on the ground, for example, birds and squirrels, as well as chase the rapidly swirling leaves. The cat is furthermore said to have very defective daylight vision and to be colorblind (DeVoss and Ganson, '15). The alligator hunts along horizontal contours, and yet one finds that the shape of the pupil is a vertical slit.

SUMMARY

1. The eye of the alligator possesses a well-developed retinal tapetum in the dorsal and posterior portions of the retina to within 1.5 mm. of the entrance of the optic nerve. It is formed by the inclusion of guanin in the cells of the epithelial layer (figs. 1, 2, 3, 4). The pecten consists of a slightly raised pigmented cap covering the entrance of the optic nerve.

2. Typical rods and cones occur throughout the retina, the cone-rod ratio being different for different regions, but characteristic for particular regions (p. 216).

3. The rods are all of one type (fig. 5), the cones of two, thick and thin, of which the first is by far the more numerous, occurring particularly in the posterior and ventral portions of the retina. Those of the second type are found only in the ventral portion and are not numerous (fig. 6). Double cones also occur (figs. 7 and 8). None of the cones contain oil drops. The rod nuclei are oval or elliptical in shape, and the majority of them project through the external limiting membrane for a variable extent, the rest of them being just under it. The cone nuclei are pear shaped and, occupying a deeper level than the rod nuclei, constitute a second row (figs. 5, 6, 9, 10).

4. The rods show a change in length of their myoids averaging 4μ, being longer in the light and shorter in the dark (figs. 9, 10, 11 and table 1). The single cones (thick and thin) show an average change of 2.1μ (figs. 11, 12, 13 and table 1). The

smaller member of the double cones shows an average change in length of 3.5μ, the larger member of 2.7μ (figs. 7, 8 and table 1). 5. The actual change in position of the pigment between light and dark eyes is slight, averaging but 1.6μ; but when combined with the change in length of the visual cells, gives an effective migration equal to the sum of the two (figs. 9, 10, 11).

6. A theoretical consideration of photomechanical changes and of the duplicity theory from a comparative point of view is appended.

BIBLIOGRAPHY

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64 (1646)

Paramecium calkinsi sp. n.

By LORANDE LOSS WOODRUFF.

[From the Osborn Zoölogical Laboratory, Yale University.]

There are at present four well-established species of Paramecium (P. aurelia, P. caudatum, P. bursaria, and P. putrinum) which fall naturally into two quite clearly defined groups. One group comprises Paramecium aurelia and Paramecium caudatum which are characterized by a relatively long spindle-shaped body. The other group includes Paramecium bursaria and Paramecium putrinum which exhibit a somewhat shorter and broader form, with a tendency toward a dorso-ventral flattening. All the species have a single micronucleus except Paramecium aurelia, which has two micronuclei each showing characteristic 'endosome' structure.1

The purpose of the present paper is to record the discovery in January, 1920, of a hitherto undescribed form of Paramecium. It has now been extensively studied in pedigree cultures for more than a year and during this time it has bred true. I therefore definitely designate it a new species, Paramecium calkinsi, in recognition of the fact that Professor G. N. Calkins of Columbia. University introduced students of the Infusoria to exact, daily isolation, pedigree culture methods.

The general body form of the new species places it at once in the bursaria-putrinum group, but its micronuclei both in structure and number are identical with those of Paramecium aurelia. In brief, Paramecium calkinsi represents the 'aurelia' type of micronuclear complex in the 'bursaria' group of species.

Details of the structure and life history of Paramecium calkinsi will appear in the Biological Bulletin.

1 H. S. Jennings and G. T. Hargitt: "Characteristics of the Diverse Races of Paramecium," Journal of Morphology, 1910, xxi, 495. L. L. Woodruff: "Paramecium aurelia and Paramecium caudatum," Journal of Morphology, 1911, xxii, 223.