260 



PTERIDOPHYTA. 



[CH. 



margin of the coherent segments ; the teeth are deciduous, and 

 after they have fallen the sheath presents a truncated appear- 

 ance. This difference between the sheaths to which the teeth 

 are still attached and those from which they have fallen is 

 illustrated by fig. 58, B and C ; it is one which should be borne 

 in mind in the description of fossil species, and has probably 

 been responsible for erroneous specific diagnoses. In some 

 recent Horse-tails the sheath is occasionally divided in one or 

 two places by a slit reaching to the base of the coherent 

 segments' ; this shows a tendency of the segments towards the 

 free manner of occurrence which is usually considered a Cala- 

 mitean character. In certain fossils referred to the genus 

 Annuluria, the nodes bear whorls of long and narrow leaves 

 which are fused basally into a collar (fig. 58, B). There are 

 good grounds for believing that at least some Annularias were 

 the foliage shoots of true Calamites. Again, in some species of 

 Galanntinu, a sub-genus of Calamites, the leaves appear to have 

 been united basally into a narrow sheath. We see, then, that 

 it is a mistake to attach great importance to the separate or 

 coherent character of leaf-segments in attempting to draw a 

 line between the true Calamites and Equisetites. Potonie- 



Fig. 56. Calamitean leaf-sheath. From a specimen in the Woodwardian 

 Museum, a, base of leaf-sheath ; (very slightly reduced). 



1 PotoniS (93) PI. XXV. fig. la. 



^ iUd. p. 179. Vide also Potonie (92). 



