X] ARTHROPITYS. 325 



Geology, is historically interesting as being one of the first 

 important plants obtained by Williamson early in the fifties, 

 when he began his researches into the structure of Carbo- 

 niferous plants. A joiner, who was employed by Williamson 

 to make a piece of machinery for grinding fossils, brought a 

 number of sandstone fragments as an offering to his employer, 

 whom he found to be interested in stones. The specimens 

 "were in the main the merest rubbish, but amongst them," 

 writes Williamson, " I detected a fragment which was equally 

 elegant and remarkable... In later days, when the specimen so 

 oddly and accidentally obtained, came to be intelligently studied, 

 its history became clear enough, and the priceless fragment 

 is now one of the most precious gems in my cabinets" 



Comparison of three types of structure met with in Calamitean 

 stems, — Arthropitys, Arthrodendron, and Calamodendron. 



The anatomical features which have so far been described 

 as characteristic of Catamites represent the common type met 

 with in the English Coal- Measures. The same type occurs also 

 in France, Germany and elsewhere. It is that form of stem 

 known as Arthropitys, a sub-genus of Catamites. 



Arthropitys may be briefly diagnosed as follows, — confining 

 our attention to the structure of the stem : A ring of collateral 

 bundles surrounds a large hollow pith, each primary xylem 

 strand terminates internally in a more or less bluntly rounded 

 apex traversed by a longitudinally carinal canal. The principal 

 medullary rays consist of large-celled parenchyma, of which the 

 individual elements are usually tangentially elongated as seen 

 in transverse section, and four or five times longer than broad 

 as seen in a tangential longitudinal section. The secondary 

 xylem consists of scalariform and reticulately pitted tracheids ; 

 the interfascicular xylem may be formed completely across each 

 primary ray at an early stage in the growth of the stem*, or 

 it may be developed more gradually so as to leave a tapering 



1 Williamson (96), p. 194. 



2 Vide specimens 15—17, etc. in the Williamson Collection. 



