CONIFERS. 187 



distichum, Kich. ; also, to a certain extent, in Sequoia sempervirens, 

 Endl., Thuja occidentalis, L., Lilocedrus decurrens, Torr., etc. 1 ; but 

 in most species the leaves remain on the tree for more than one 

 year. Occasionally, the manner of occurrence of detached leaves or 

 leafy shoots in a fossil state may afford evidence of the existence of 

 deciduous species. A. careful examination of branches of recent 

 conifers bearing vegetative leaves, enables us to realize the im- 

 possibility of relying for accurate determination or comparison on 

 such uncertain characters as leaf form or arrangement. The tini- 

 veined leaves of Podocarpus in some forms of the genus, may be 

 confused with the foliage of araucarian species, in which the veins 

 are imperfectly preserved ; in such a plant as Podocarpus andina, 

 Popp., the long narrow leaves agree closely with those of some 

 forms of Cephalotaxus, and the detached leaves of either bear a strong 

 resemblance to single pinnae of Cycas. A specimen of an unnamed 

 species of Cephalotaxus in the British Museum possesses leaves 

 measuring 11 cm. in length and 4mm. broad, a close approach to 

 the pinnae of Cycas species. The genus Agathis, e.g. A. Australia, 

 Salisb., cannot be readily distinguished from some forms of Arau- 

 caria, if we have only the leaves to guide us. The large leaves of 

 Agathis Dammara, Rich., and the broad pinnae of Podozamites 

 cannot always be separated with certainty, at least in such 

 specimens as do not show distinct venation. If we have not the 

 general habit of the tree, or characteristic differences of colour to 

 help us, it is practically impossible to discriminate with accuracy 

 between the leafy twigs of many recent genera. Without entering 

 into any detailed comparison of living forms from this point of 

 view, we may note the close agreement between Taxodium distichum, 

 Rich., Sequoia sempervirens, Endl., and Taxus baccata, L. ; between 

 Cryptomeria, sp., and Araucaria, sp. ; between different genera of 

 the Cnpressince, etc. Similar examples might be readily multiplied ; 

 but an examination of the recent species will at once demonstrate 

 the futility of attempting generic distinction on such data, and will 

 emphasize the unfortunate habit of some writers of applying to 

 fossil fragments the unaltered names of recent genera. Another 

 pitfall as regards leaf form, is the fairly common occurrence of 

 heterophylly among coniferous plants. Writing in 1803, Lambert 



1 Stark. 



