XXXIV] MESOPITYS 295 



out, as in Poroxylon, constitutes a difference from Lyginopteris 

 in which the division occurs later. 



Zalessky's generic name' Parapitys is an appropriate substitute 

 for Dadoxylon in view of the presence of separate primary xylem 

 strands, a feature foreign to typical Dadoxylons which agree 

 with recent Conifers and Cycads in the absence of vascular strands 

 distinct from the endarch centrifugal wood. As Sdott^ says, 

 Parapitys 'is best regarded as a near ally of Mesoxylon.' 



Mesopitys. Zalessky. 



Mesopitys Tchihatcheffi (Goeppert). The genus Mesopitys 

 was instituted by Zalessky for a Permian species founded by 

 Goeppert^ on a piece of decorticated stem from the Kousnetzk 

 basin in Siberia and afterwards more fully described by Goeppert 

 and Stenzel*. The structure of the secondary wood led Goeppert 

 to adopt the name Araucarites for which Kraus* substituted 

 Araucarioxylon. A recent investigation of additional material 

 by Zalessky brought to light the existence of groups of primary 

 xylem abutting on the secondary xylem and projecting into the 

 pith, characterised by the occurrence of spiral protoxylem elements 

 on the inner edge. The recognition of this important feature 

 justified Zalessky in the adoption of a new generic term. In 

 general anatomical characters Mesopitys agrees with Eristophyton 

 Beinertianum (Goepp.) but is distinguished by the more feebly 

 developed primary-xylem groups and by their endarch structure. 



The examination of sections from some of Prof. Zalessky's 

 material lent to me by Dr Kidston enables me to confirm 

 Zalessky's description, though I am not convinced that the primary 

 xylem-strands are exclusively endarch: in most of the primary 

 groups the protoxylem is clearly on the inner edge, but in a few 

 cases there may be a small amount of centripetal xylem present. 

 The characters of Mesopitys Tchihatcheffi may be summarised 

 as follows: 



Annual rings well defined, varjring considerably in breadth; 

 the summer wood is represented by several rows of narrower 



1 Scott (12) p. 1028. 



2 Goeppert (45^) p. 11, Pis. xxix. — xxxv. ; Zalessky (11). 



3 Goeppert and Stenzel (88) p. 33, Pis. v., vi. 

 1 Kraus in Schimper (72) A. p. 381. 



