DESCRIPTION OF SPECIES— DICOTYLEDONOUS LEAVES. 123 



or is found explained in text-books, especially in Gray's Lessons in Botany. 

 But that which relates to nervation forms a separate section, considered only 

 in works on vegetable palaeontology, rarely accessible to the student. I there- 

 fore give herewith an abridged explanation of the terms which may be used 

 ill describing the American Tertiary plants in this memoir. For some authors, 

 as for d'Ettingshausen, who has given to the subject a careful and long study, 

 the groups fixed by the characters of the nervation are numerous, and their 

 distinction is somewhat embarrassing in many cases. Heer's classification is 

 by far the simplest, and more comprehensible. It has been admitted by 

 many European authors, especially by Schimper in his Paldontologie V(ig^tale. 

 According to it, we have : — 



1. Leaves penninerved. — These have a midrib, or primary nerve, 

 brandling on each side. These branches, the secondary nerves, have to be 

 considered in their position, as alternate or opposite, in their respective dis- 

 tance, and especially in their angle of divergence, from the middle nerve and 

 their direction toward the borders. Their branches are tertiary nerves, or 

 veins, except when they join the secondary nerves, either immediately or by 

 subdivisions, when they become nervilles or veinlets. When these veinlets 

 are in right angle to the secondary or tertiary nerves, and pass across them, 

 they are percurrent nerviUes; when they are curved, or broken, or connected 

 with veinlets of the same order, they become ivjlected or broken nerviUes. 



According to the distribution of the nerves, the surface of the leaves is 

 divided in are^s of different orders. Those which are limited on one side by 

 the primary nerves, and fill the space between two secondary ones, are named 

 areas of the first order. Those which are surrounded by secondary and ter- 

 tiary veins are areas of the second order. When the veinlets divide in thinner 

 branches, they surround areola, which may be subdivided by descriptions in 

 areola or meshes of the second, third, fourth order, etc. Sometimes the primary 

 areas are traversed, as in the leaves of Willows, by shorter secondary nerves, 

 which soon divide into areolae or join by their branches the secondary nerves 

 of tiie first order. These are generally named tertiary nerves, or shortened 

 secondary, or pseudo-secondary nerves. The areas of the different orders are 

 often clearly defined, as in the leaves of the Maple, for example ; but some- 

 times they gradually become undiscernible by the thinning of the veinlets, 

 and lose themselves in the netting. Li this case, the ultimate <livisions are 

 called dissolved nervilles. 



