346 ON LYELLIA, LEPTOSTOMUM,
sought for, or those at present in use more minutely inves- tigated.
Of additional characters, which in some cases may be employed with advantage, I shall merely advert to the membranes of the capsule being distinct or contiguous, and to that more intimate union where there seems to be a single membrane only; to the modifications of internal structure of the inner membrane; the differences in form and duration of the columella, or even its being entirely wanting in the ripe capsule ; the presence or absence of 577] the annulus; and the insertion, form and relative position of the male flowers, which, though always con- sidered of importance by Hedwig, many of the most dis- tinguished muscologists of the present day entirely exclude from the characters of their genera.
With respect to the principal source of generic distinc- tions, the Peristomium, in addition to the circumstances generally attended to, namely, the origin, number, direction, form, and actual division of the teeth, it may be of some importance to ascertain their aestivation, which, though very generally, is not always valvular : and especially to mark the existence or want of the longitudinal stria or semi-pellucid lines : for these, if they do not prove the compound nature, at least clearly indicate a tendency to division in the teeth where they are found ; division being always in the course of the striae, and in no instance taking place unless where they are present.
But in considering them, which I am inclined to do, as proving composition or confluence of the teeth, it would appear that there is a much greater uniformity in the struc- ture of the simple or outer peristominm, at least, than is generally admitted; and that the prevailing number of teeth in this series is thirty-two ; though by a coalescence, more or less complete, they are frequently reduced to six- teen, in some cases to eight, and in a few 7 even to four.
According to this view, a single longitudinal line in the axis of a tooth indicates the confluence of two teeth ; three equidistant lines, one being central, the coalescence of four ; and seven lines similarly disposed that of eight.
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