the above data is 0.0024. This leaves a decrease of 0.0127 to be accounted for by the increase of pressure of 2640 kgm. which gives a mean compressibility over this range of 0.0000048, a little more than one third of the compressibility of the liquid over the same range.

The direct determination of the compressibility of the ice was made by two different methods. One of these was the same as that used roughly in the preceding paper, that is by finding the difference of the slope of the curves plotting piston displacement against pressure above and below the transition point to the solid. The values obtained in the preceding paper for this were very rough. In these determinations the cylinder was very much more carefully seasoned, and the readings were made with all the precautions which had been suggested by all the experience of this paper. Two determinations of this quantity were made at 0° and also two determinations at 20°. The two values for the difference of compressibility differed by 2.5% at 0° and by 0.7% at 20°. The value found for the difference was 0.0000087 at 0° and 0.0000067 at 20°. Combining with the values given already for the compressibility of the liquid, this gives for the compressibility of ice VI 0.0549 at 0° and 6360 kgm., and 0.0543 at 20° and 9000 kgm. Mean 0.0546.

The second method for determining the compressibility was exactly the same as that for finding the same quantity for the liquid, comparing the displacements when the apparatus was filled with ice and kerosene with those when the ice was replaced by bessemer steel. This determination was made over a wider pressure range, to find if possible the variation of compressibility with pressure. No variation with pressure could be found over a range of 4500 kgm. at 0° and 3300 kgm. at 20°. The absolute values do not agree with those found by the two other methods, however, the figures being 0.0531 at 0° and 0.0535 at 20°. The cause of the discrepancy is not clear, but is probably connected in some way with the hysteresis of the cylinder. The hysteresis was not regular for these small pressure ranges, being at times almost negligible, and again being as large as for almost the entire pressure range from atmospheric pressure to the maximum. There seems little question but that the greater weight is to be attached to the values found by the first two methods. This third determination does show, however, that the variation of the compressibility with pressure and temperature over this range is so small as to be beyond the accuracy of these measurements. In selecting the best probable value for the compressibility the only weight that will be

assigned to this third determination is in slightly lowering the mean of the other two.

The final most probable values for Ice VI are as follows: for the compressibility 0.0545, and for the thermal dilatation 0.000120 cm.3/ gm. over the range 6360-10,000 kgm. and 0° to 20°.

The cost of much of the apparatus used in this investigation was defrayed by an appropriation from the Rumford Fund of the American Academy.

JEFFERSON PHYSICAL LABORATORY,

HARVARD UNIVERSITY, CAMBRIDge, Mass.

Proceedings of the American Academy of Arts and Sciences.

VOL. XLVIII. No. 10.-SEPTEMBER, 1912.

CONTRIBUTIONS FROM THE CRYPTOGAMIC LABORATORY OF HARVARD UNIVERSITY.

LXXI. — PRELIMINARY DESCRIPTIONS OF NEW SPECIES OF RICKIA AND TRENOMYCES.

BY ROLAND THAXTER.

CONTRIBUTIONS FROM THE CRYPTOGAMIC LABORATORY OF HARVARD UNIVERSITY.

LXXI. PRELIMINARY DESCRIPTIONS OF NEW SPECIES OF RICKIA AND TRENOMYCES.

BY ROLAND THAXTER.

Received August 19, 1912.

RICKIA.

THE genus Rickia has proved to be a large and varied one, and although I have enumerated below only those forms parasitic on Acari which have come under my notice, many others are known to me on a variety of hosts, an account of which I have reserved for a future paper. The general habit appears to be very variable, including in addition to the condition seen in the type form, others in which the median cell-series is undeveloped, as well as various species with a more or less complicated system of branches. The antheridial characters, moreover, appear to be equally variable. Not only do the antheridia which are extraordinarily abundant in some species seem wholly lacking in others, but their character may vary in different cases. In some there may be a single antheridium, only, similar to that of Peyritschiella, definitely placed at the base of the perithecium; or an antheridium of this type may be associated with others of the normal habit variously disposed. Again even in forms having the three characteristic cell series, antheridia may be present like those of the genus formerly separated as Distichomyces, each antheridial cell becoming more or less free in a compact group. Since both the antheridial characters and those of the receptacle thus appear to be so variable, it has not seemed desirable to limit the genus to the type form as illustrated by Rickia Wasmanni, and I have therefore given it a more liberal interpretation; including under it forms with two or with three cell-series, whether they be simple or branched, and whether their antheridia be of the Rickia or the Distichomyces type. The latter genus is, therefore, abandoned, one species only, Rickia Leptochiri, being involved in this change.