392 HISTORY OF GEOLOGY AND PAL/EON TOLOGY. 



development, the growth, relations, and geological distribution 

 of the Graptolites. All later writings on Graptolites are based 

 upon the results obtained by Lapworth. During the last few 

 years Holm and Wimann have by means of novel methods 

 of technique determined the finest structural features of different 

 Graptolite genera, and R. Riidemann (1895) made some for- 

 tunate discoveries which threw light on the mode of life and 

 the relationship of these remarkable organisms. 



Fossil Medusas are of very rare occurrence ; well-marked 

 impressions found in the lithographic shales of the Franconian 

 Jura Chain have been carefully described by Beyrich (1849), 

 Haeckel (1865-70), and Ammon (1883). Nathorst in 1881 

 assigned to the Medusas certain casts in the Cambrian sand- 

 stone of Sweden, and quite recently (1898) Walcott described 

 a large number of cast structures in the Cambrian deposits 

 of North America as of Medusa origin. 



Echinoderms. In the eighteenth century Klein had proposed 

 for the sea-urchins the class name of Echinodermata. Cuvier 

 united under the same class the Ophiuridea or sand-stars, 

 the Holothuridea or sea-slugs, and the Encrinites^ without, 

 however, recognising the Encrinites as a separate sub-division. 

 In 1821, J. S. Miller, a native of Dantzig although resident in 

 Dublin, published an excellent monograph on all the fossil 

 Sea-lilies or Encrinites then known, and combined them into 

 an independent sub-division or order which he named 

 Crinoidea. In 1828, Fleming erected the order of Blasioidea 

 for the Pentremites which had been discovered in 1820 by Say 

 in the North American Carboniferous Limestone, and in 1845 

 Buch erected the order of Cystidea for a group of fossil 

 Crinoids then very little known. Thus the limits and the 

 chief orders of the Echinodermata were definitely established, 

 and at the suggestion of Leuckart in 1848 the class Echino- 

 dermata, which had hitherto been treated systematically as 

 closely allied with the Ccelenterata, was represented as an 

 independent branch of descent in the animal kingdom. In 

 addition to Leuckart's fundamental differentiation of these two 

 animal classes, it was he who first combined the Crinoidea, 

 Cystoidea, and Blastoidea under a common group -name 

 Pelmatozoa,) and placed it in contradistinction to the other 

 sub -divisions of Echinodermata, the Echinidea, Asteridea, 

 Ophiuridea, and Holothuridea. 



