Maryland Geological Survey 309 



Equisetites). Their internal structure is' unknown, and as some of the 

 Calamariaceae had leaves united in sheaths the remains of foliage from 

 the older rocks is not unequivocal. The cone impressions, however, are 

 not so easily disposed of, and it seems quite possible that the later 

 species form a continuous line of descent from some similar herbaceous 

 Paleozoic stock which was plastic enough to adapt itself to varying sub- 

 sequent conditions. 



With the opening of the Mesozoic age forms clearly allied to the re- 

 cent genus are common fossils. These are largely stem or pith casts (de- 

 tached diaphragms and tubers are also fossilized) and are often of large 

 size, i. e., from 12 to 15 cm. in diameter and 8 to 10 m. tall, quite as 

 tall as the largest living species and not nearly so slender. It seems 

 quite possible that some of these Triassic and Jurassic forms may have 

 increased in diameter by the formation of secondary wood, as did so 

 many of their Paleozoic allies. 



The later Mesozoic species of Equisetum are much smaller than their 

 Triassic and Jurassic ancestors, and are scarcely distinguishable from 

 existing forms. These latter, commonly known as " horse tail," " mare's 

 tail,'' or "scouring rushes," form a compact group of from twenty-five 

 to thirty rather monotonously uniform species, growing usually in low 

 and wet habitats and present on all the continents except Australia, 

 although they are found as fossils in that region. They are present in 

 tropical and arctic as well as in the temperate zones, and the majority 

 are of small size, although one species, Equisetum giganteum. Linne, 

 which ranges from Cuba to Chile, is said to attain a height of about 

 8 m., but remains very slender and not over 2 or 3 cm, in diameter. 



The Potomac forms are not important elements in the flora, although 

 they are present in considerable abundance at somei localities, nor are 

 they at all well preserved. There are no apparent grounds for maintain- 

 ing them as distinct from the rather widespread European forms of this 

 ■ age, and they are therefore identified with the latter, whose geographical 

 ranges are quite comparable to that of such ubiquitous existing forms as 

 Equisetum arvense Linne. 



