12 Contributions towards Establishing the General Character 



establish this in a positive manner ; for there are numerous differences 

 between Sigillaria elegans and the plants at present known as belonging 

 to the last division. For example, 



Istj No knov/n Conifer nor Cycas possesses the medullary sheath of 

 Sigillaria clegansj as regards its form and the non-radial arrangement of 

 its constituent vessels. 



2d, In no Conifer is the ligneous cylinder composed of tubes or elon- 

 gated cells transversely barred ; on the contrary, coniferous wood is 

 formed of tubes characterised with disks, and these only on the walls 

 parallel to the direction of the medullary rays. 



3i, Amongst the Cycases, Zamia integrifoUa is the only known plant 

 possessing vessels which are barred or reticulated on all their walls ; but 

 this character is not so regular as in the fossil under consideration.* 



Thus, between Zamia integrifolia and Sigillaria elegans, there is a strong 

 point of agreement. It requires to be mentioned, however, that the 

 former does not possess the peculiar medullary sheath of the latter, nor 

 are the vessels of the ligneous cylinder so regularly arranged in radiat- 

 ing series. 



Brongniart, in the next place, refers to the Cactuses and the Euphor- 

 bias which constitute two remarkable families of the Angiospermous di- 

 vision of the Dicotyledons. The genera Melocactus and Echinocactus are 

 particularly noticed. From transverse sections it appears, that between 

 these plants and Sigillaria elegans there is a remarkable general resem- 

 blance,iparticularly in the presence of apparently isolated vascular bundles 

 within the central column of cellular tissue or pith, as displayed in one of 

 the species. An examination, however, of the encircling zone shews an 

 important difference between this part and what has been called the lig- 

 neous cylinder of the fossil ; it is, in fact, formed of different tissues, 

 namely, true spiral vessels, ducts, and a peculiar ligneous fibre. Hence 

 Brongniart is of opinion that Sigillaria elegans cannot, with any propriety, 

 be considered a Cactus. The same objection may be urged against its 

 being considered an Euphorbia. 



" Thus," to conclude this abstract in the language of Brongniart, '' our 

 researches among the Angiospermous Dicotyledons do not enable us to 

 discover any decided analogy between these plants and Sigillaria. Ought 

 vre, therefore, to assert positively that no analogy does exist } I think 

 not, for the variety of characters in the organization of the stems of these 

 plants ought to induce one to suppose that we shall yet find among them 



* There is every reason to believe that in this respect the Jurassic Cycases 

 approximated more to Sigillaria elegans than Zamia integrifolia. It is stated in 

 Dr Buckland's Bridgewater Treatise, that Dr Brown has discovered in the genus 

 Mantellia only scalariforra vessels : — " The existence of vessels with discs pecu- 

 liar to recent Cycadece and Coniferce, has not yet been ascertained." — (Vol. i. p. 99, 

 foot note; and vol. ii. p. 100, foot note). Mr Morris of Kensington has kindly 

 placed in my hands some sections of Cycadeous wood from the Jurassic rocks of 

 Yorkshire, and I find the vessels are marked with transverse bars very similar 

 to those of Sigillaria elegans. 



