XVI INTRODUCTION. 



The peristome may be single, i.e., may form a single circle 

 or fringe (Tab. V. 6, 17) ; or it may be double, consisting of two 

 concentric circles, an outer and an inner (Tab. V. 7, 12) ; very 

 close however to one another, and sometimes even partly united. 

 Taking the single peristome first, it sometimes consists of a thin 

 membrane, rising conically over the mouth of the capsule, leaving 

 only a minute aperture at its apex; much more frequently 

 however, indeed in the vast majority of cases, the membrane is as 

 it were split from top to bottom into a number of filaments, called 

 the peristome teeth. These teeth are always produced in multiples 

 of 4, indeed there are always either 4, 8, 16, 32, or 64, the median 

 numbers being by far the most frequent. They vary very greatly 

 in length, form, colour, and other characteristics, red being the 

 predominant tint. They are usually very thin in texture, in most 

 cases consisting of a double layer of very thin plates, divided 

 transversely into segments, with more or less distinct lines at the 

 points of division, sometimes forming very strong trabecules, or 

 bars, standing out prominently on the face of the tooth like the 

 rungs of a ladder (Tab. V. 10). 



The double peristome is more complicated in structure. The 

 outer fringe superficially resembles the single peristome already 

 described, although differing in minute points of structure ; the 

 inner peristome is always more delicate in texture, usually paler 

 in colour ; occasionally it consists of 8 or 16 delicate filaments or 

 processes alternating with the outer teeth, more rarely opposite 

 them ; in other groups these processes are wider, meeting one 

 another at their base ; while still more often they are more or 

 less united below, so as to form a continuous basal membrane 

 surrounding the orifice, while their upper portions remain free ; 

 these upper portions being frequently more or less split or 

 pierced along the median line. The spaces left between these 

 free upper portions are often occupied by still more slender 

 prolongations of the basal membrane, termed cilia, singly, or 

 grouped in twos or threes ; these are extremely delicate and 

 thread-like, and frequently in the most highly developed forms 

 bear short transverse appendages at intervals (Tab. V. 15, 16). 



All these different parts exhibit much variation in the degree 

 of their development, and the whole peristome is often reduced to 

 a fringe of short rudimentary teeth around the orifice ; the teeth 

 are often, also, on the one hand cleft, to a greater or less extent, 

 into two or three branches, or again, more or less approximated 

 or even united in pairs (Tab. V. 17), or in fours, or they may be 

 connected, in various degrees, by transverse bars, so as to form a 



