556 Notices of Memoirs-^ 



extends into the Wealden period. Ifc is a remarkable fact that 

 after the Palaeozoic floras had been replaced by those of the 

 Mesozoic era, the vegetation maintained a striking uniformity of 

 character, from the close of the Triassic up to the dawn of the 

 Cretaceous era. 



Mesozoic Floras. — It may be of interest to glance at some of the 

 leading types of Mesozoic floras with a view to comparing them with 

 their modern representatives. We are so familiar with the present 

 jiosition of the flowering plants in the vegetation of the world, that 

 it is difficult for us to form a conception of a state of things in the 

 history of the plant-kingdom in which Angiosperms had no part. 



A. Conifers. — How may we describe the characteristic features of 

 Ehgetic and Jurassic floras? Gymnosperms, so far as we know, 

 marked the highest level of plant-evolution. Conifers were 

 abundant, but the majority were not members of that group to 

 which the best known and most widely distributed modern forms 

 belong. 



A comparison of fossil and recent conifers is rendered difficult by 

 the lack of satisfactory evidence as to the systematic position of 

 many of the commoner types met with in Mesozoic rocks. There 

 are, however, certain broad generalisations which we are justified in 

 making ; such genera as the Pines, Firs, Larches, and other members 

 of the Abietinese appear to have occupied a subordinate position 

 during the Triassic and Jurassic eras; it is among the relics of 

 Wealden and Lower Cretaceous floras that cones and vegetative 

 shoots like those of recent Pines occur for the first time in a position 

 of importance. There are several Mesozoic Conifers, to which such 

 artificial designations as Fagiopliyllum, Brachyphyllum, and others 

 have been assigned, which cannot be referred with certainty to 

 a particular section of the Coniferse ; these forms, however, exhibit 

 distinct indications of a close relationship with the Araucariese, 

 represented in modern floras by Araucaria and Agathis. The 

 abundance of cones in Jurassic strata showing the characteristic 

 features of those of recent species of Araucaria affords trustworthy 

 evidence as to the antiquity of the Araucarieas, and demonstrates 

 their wide geographical distribution during the Mesozoic era. 

 Additional confirmation of the important status of this section of the 

 Coniferee is afforded by the abundance of petrified wood exhibiting 

 Araucarian features, in both Jurassic and Wealden rocks. There is 

 good reason to believe that the well-known Whitby jet was formed 

 by the alteration of blocks of Araucarian wood drifted from forest- 

 clad slopes overlooking a Jurassic estuary that occupied the site of 

 the moors and headlands of North-East Yorkshire. 



B. Cycads. — One of the most striking features of the Mesozoic 

 vegetation is the abundance and wide distribution of Cycadean 

 plants. To-day the Cycads or Sago-Palms are represented by ten 

 genera and about eighty species ; they are plants which occupy 

 a subordinate position in modern floras, and occur for the most part 

 as solitary types in tropical latitudes, never growing together in 

 sufficiently large numbers to constitute a dominant feature in the 



