706 A CENTURY OF PROGRESS IN THE NATURAL SCIENCES 



a systematic description of all known Paleozoic forms. He assumes that any 

 scientific classification should be based on the totality of characters and not on 

 single phases of the morphology. Emphasis is laid on the use of stages in de- 

 velopment and a comparison of these with the characters of more or less closely 

 associated types. He developed the idea of localized stages in development, the 

 idea that, "throughout the life of the individual, stages may be found in definite 

 parts that are comparable to the condition in the young and to adults of simpler 

 types of the group." This principle was found applicable to such other groups 

 as Crinoids, Corals, and Cephalopods and has been used by Hyatt, Beecher, 

 Cushmann, Kuedemann, H. L. Clark, and many others. Jackson points out that 

 senescence is well shown in Paleozoic Echini by the dropping out of columns of 

 interambulacral plates at the dorsal portion of the test. He defines progressive 

 types as "those which show in their development to maturity the addition of dif- 

 ferential characters only ..." without their later disappearance and points out 

 that most Paleozoic Echini are of this type. Regressive types are considered to 

 be those which show specialized characters in later development but lose these 

 before old age so that the adult is simpler than its ow^n young. Other characters 

 discussed by Jackson are acceleration of development, parallelism, and variation. 



The more important factors used by Jackson in considering the comparative 

 morphology of the Echini were the form of the test, the pentameral system, the 

 structure of the skeleton, the ambulacra and interambulacra of the corona, the 

 spines, peristome, and occular and genital plates. His classification of the Echi- 

 noidea consists of 7 orders and 17 families. 



After a critical study of plate structure, Sven Loven in 1874 devised a nomen- 

 clature for referring to the ambulacra in terms of the Roman numerals I to V 

 and the interambulacra by the Arabic numerals 1 to 5. He established the 

 method of determining the bilateral symmetry of the test by the presence of 

 the madrejjorite or the periproct. 



Important contributions were made to the Mesozoic and Tertiary Echinoids 

 in North America by W. B. Clark in 1893, W. B. Clark and M. W. Twitchell 

 in 1915, W. S. W. Kew in 1920, and Grant and Hertlein in 1938. Cenozoic Echi- 

 noids have been described by P. M. Duncan from Australia and by Duncan and 

 Sladen from the Western Sind of India. The papers by J. Lambert and P. 

 Thiery (1909-1925) are important for their taxonomic work on the Echinoidea. 



Fossil starfish are not extensively used for purposes of geological correlation, 

 yet they occur in rocks from the Ordovician to the present. Many papers have 

 been written describing genera and species, among the more important of which 

 are those of Sladen and Spencer from 1890 to 1908 on the British fossil Asteroi- 

 dea, F. Schondorf from 1907 to 1913 on the German Paleozoic forms, and a 

 monograph by Charles Schuchert published in 1915 on the Paleozoic Stelleroi- 

 dea. In this volume there are recognized 45 genera and 110 species. Fifty-one of 

 the species are from North America, 53 from Europe, and 6 from the southern 

 hemisphere. This paper is of special importance for its presentation of the skele- 

 tal terminology of the Asteroidea. 



Brachiopoda : The class name Brachiopoda was proposed in 1802 by Cuvier. 

 A memoir by von Buch in 1834 contained a classification based largely on the 

 characters of the hinge area. The character of the brachial appendages, the 

 septum, the muscular impressions, and other internal structures were used by 



