264 PROF. W. C. WILLIAMSON ON A NEW FORM 



idea tliat Calamopitus was Equisetaceous ; on the other 

 hand, whilst the stem was more complex than that of living 

 Equisetacesej the fossil spores are more simple in their or- 

 ganization than in the recent genus. The frnit sustains 

 the conclusion at which I arrived from a study of the stem, 

 that Calamopitus possessed a higher organization than the 

 known forms of C alamo dendr on. 



The exogenous growth of the stem by the regular addi- 

 tion of new vascular bundles to the exterior of the woody 

 axis, indicates the possession by these plants of a cambium- 

 layer such as exists in the living Marsileacese. Unfortu- 

 nately we know too little of the cortical layer of Calamites 

 to affirm any thing respecting it j but it becomes of great 

 importance to ascertain whether it was persistent, receiving 

 internal additions from the cambium-layer (in which case 

 we should expect to find it in some degree ruptured exter- 

 nally) , or whether it was thrown off annually and annually 

 reproduced as in the living Isoetes. As yet we possess no 

 facts throwing light upon either of these problems'^ ; but 

 when we obtain their solution, we shall doubtless find in it 

 the explanation of the great differences observable, both 

 in the aspect of the cortical layer of fossil Calamites and 

 in the opinions of authors respecting its thickness and 

 aspect. 



Mr. Butterworth informs me that he found the specimen 

 here described in a nodule from the upper foot-coal at 

 Roe Buck, in Strinesdale, Saddleworth. 



INDEX TO PLATES VII., VIII., & IX. 



Fig. I. Lateral aspect of the lowest segment of the specimen, enlarged 



four diameters. 

 Fig. 2. Inferior surface of the same segment, showing the pyriform canals 



around the medulla, a ; b, margin where the bractigerous disk 



divides into separate bracts. 



* This bark has now been obtained, and will shortly be described — 

 April 21, 1870. 



