438 Scientific Proceedings, Boyal Dublin Society. 



of a species; and the mean height of ray given by the average of 5 or JO 

 rays of greatest width, taken at random, is sometimes a useful charac- 

 teristic. For example, the maximum diameter of the vessels of Lumbayao 

 (PI. XLIV., fig. 133) and Cape Lopez (PI. XXXVII., fig. 91) mahoganies is 

 0-45 mm. and 0'50 mm., respectively, and gives us a safe distinguishing 

 characteristic when we are comparing them with the Khayas or Swietenias, 

 in which, so far as my experience goes, the diameters are never greater than 

 0'2o mm. and 0-32 mm., respectively. Turiballi (PI XXVII., fig. 34) may be 

 instanced as a wood characterized by very small vessels, their maximum 

 diameter being no more than 042 mm. [For the sake of comparison in the 

 following descriptions the measurement of the diameter is taken in a 

 tangential direction, and the maximum tangential diameter (M.T.D.) of the 

 vessels is recorded lor each specimen examined.] Similarly, the average 

 height of the 3-4 ply medullary rays of Lumbayao is l - 5 mm. (PI. XLIV., 

 fig. 134), and this characteristic readily distinguishes this wood from that of 

 the Swietenias (PI. XXIL, figs. 2, 5, PI. XXXIIL, fig. 68), where the average 

 height of the rays does not appear to exceed - 65 mm. 



Sometimes the medullary rays seem to occupy a large proportion of the 

 tangential section ; again in other Woods but a small proportion of the total 

 area of the section is composed of the section of the rays. This leads to a very 

 different appearance in the tangential. section of the two woods, which is 

 difficult to convey in description. To estimate the proportional area occupied 

 by the rays in the section would be tedious and impracticable where quickness 

 in identification is demanded ; so in the following descriptions the total width 

 of the rays traversed by a horizontal line 2 mm. long in a tangential section 

 is given. Sometimes this figure is of use in identification, e.g., the total width 

 of the rays in 2 mm. is usually about 0'50 mm. for Tabasco or Honduras 

 mahogany (PI. XXIII., fig. 8), while in Colombian (PI. XXV, fig. 23), Guate- 

 malan (PI. XXVIIL, fig. 41), and Gaboon (PI. XXXV, fig. 80) it does not 

 exceed 0'25 mm. But, on the other hand, great variations in this figure are 

 found for some woods, e.g., in Cuba it has been found to vary from - 79 mm. 

 to 0'50 mm., these being averages taken in different samples of this wood. Even 

 in different parts of the same sample great differences may be found, especially 

 in figured specimens, when the grain of the timber is disturbed by the 

 proximity of a branch, e.g., Klictya, sp., PL XXXII., figs. 62 and 63. These 

 sections were cut from opposite faces of a block just 1 cm. across. 



Numerical characters are also useful when the differences are great, but 

 they cannot be relied on as a rule where the two numbers approximate. This 

 is true for such characters as the number of rays seen in a given area of 

 tangential section,, or the number of vessels in a given area of cross-section . 



