JURASSIC FLORAS Samples of the vegetation prevalent in Jurassic times are scat tered through the rocks from the far north in Franz Josef Land and Alaska to north and central Europe, Siberia, India, Japan, North America, and in Graham Land on the borders of Antarctica. One of the best known floras of the Middle Jurassic period is that preserved in the upraised sediments of estuaries which form the moors and cliffs of east Yorkshire. In its main characters the Jurassic vegetation carried on the tradition which began in the latter part of the Triassic period and was further developed in the Rhaetic period. Many Rhaetic genera persisted ; some new types were evolved, and a comparative study has shown that while the Rhaetic facies in its broad features was maintained there are cer tain characters by which Jurassic floras can usually be recognized. A brief comparison with the present vegetation of the fern-cov ered river-banks and the shrubs and trees on the deltas in the area that is now traversed by the Yorkshire cliffs, from north of Whitby to south of Scarborough, may enable us to realize the more striking contrasts between the present and the past. Then as now there were horsetails (Equisetum) forming miniature for ests on the swampy ground : the plants were more robust and taller than their modern descendants, though they were smaller than their Triassic forebears. The reign of Schizoneura and Neocalamites was almost over : among the commoner plants were many which no longer occur in Europe : a botanist wandering over the deltas would be reminded of lands south of the equator : ferns such as Dictyophyllum and Matonidium would recall Dipteris and Matonia; he would also find species belonging to the Schizaeaceae, Gleicheniaceae, Marattiaceae, and Cyatheaceae, families no longer represented in the British flora and most abundant in warmer, southern lands. He would note the prevalence of species, some of them unfamiliar in the architecture of the fronds, agreeing in the structure of the spore-capsules (sporangia) with the royal fern (Osinunda regalis). He would also discover clumps of Thinn feldia probably occupying drier ground, the fern-like fronds char acterized by their stiffer leaflets without sporangia of the normal fern type. Passing to the taller shrubs and trees he would be struck by the abundance of stems bearing crowns of fronds of diverse form resembling in a greater or less degree the sago palms (cycads) of the tropics. On closer inspection he would find that
the cycadean plants bore shoots similar in shape to the flower heads of the globe artichoke (Cynara), some of them unisexual, others with male and female organs on a single axis instead of the male and female cones of modern cycads. The great majority of these cycadean plants belonged to the Bennettitales, a group that has long been extinct, and not to the cycads in the narrow sense. Most of them were constructed on the ordinary cycad plan as re gards the stem, but the type represented in the Rhaetic floras by Wielandiella persisted under a slightly different form into the Jurassic period. This aberrant Jurassic type was described by Dr. Thomas as Williamsoniella ; it had a comparatively slender and forked stem bearing scattered leaves (Taeniopteris) more fern-like than cycadean in form, and in the angles of the forked branches bisexual "flowers" resembling those of l'Vielandiella. He would probably look in vain for oaks, elms, sycamores, the ash, alder, and other broad-leaved trees, though he might discover some plants which would puzzle him by the association of a strange type of foliage with flowers distantly related to those of some modern angiosperms. He would see cone-bearing trees reminding him of the Norfolk Island pine (Araucaria excelsa), trees with shoots like those of cypresses, the incense cedar of America (Libocedrus), and the Arbor vitae (Thuja); some with two ranked leaves recalling the yews, the giant redwood of California (Sequoia sempervirens) and other conifers. The conifers as a whole would seem familiar, though the association in the North Temperate region of types that are now widely scattered in warmer countries would afford an interesting topic of speculation on the vagaries of plant-dispersal. He would be surprised to find many trees exhibiting clear signs of relationship to the maidenhair tree (Ginkgo). From his knowledge of present-day vegetation he would have difficulty in assigning to their position in the plant-world such genera as Sagenopteris, Thinnfeldia and other legacies from older floras.