504 THE AMERICAN NATURALIST [Vol. XLVII 



the Edwardsia, for the following reasons. In a semi- 

 desiccated synaptid the longitudinal lines marking the 

 longitudinal muscles become gradually obsolete, and the 

 digestive tube, more or less distended with inorganic 

 matter, becomes more and more prominent, so that finally 

 we see an elongated worm-like object with a prominent 

 digestive tube which bears a collar about the anterior 

 end; longitudinal markings become obliterated and lost 

 in the irregular foldings which take place, while these 

 also obscure the other more flaccid internal organs; a 

 soft mud-living actinian, on the other hand, is reinforced 

 internally by numerous mesenteries ; on desiccation 

 these tend to lie flat, and to raise the body wall at the 

 lines of attachment slightly, giving a fluted or pleated 

 appearance, just as is shown in Mackenzia. No trace of 

 a tubular digestive tube is visible in Mackenzia, nor is 

 there any other internal differentiation, but there are 

 prominent, regular and numerous parallel pleats; these 

 pleats are four in number in the upper part of the body, 

 but apparently five in the lower part, so that there are 

 probably eight mesenteries represented ; there appear to 

 have been sixteen tentacles, two in each intermesenterial 

 space ; the contracted lower portion of the body suggests 

 the physa of Edwardsia; it is probable, therefore, that 

 Mackenzia is an actinian, and that it should be placed in 

 the family Edwardsiidse near the genus Edwardsia. 

 Some of the preserved specimens of Edwardsia farinacea 

 in the National Museum collection are almost identical 

 with the specimen of Mackenzia costalis figured by Dr. 

 Walcott, the similarity of the contracted anterior portion 

 of the body being especially striking. 



Systematic Position of the Genera Discussed.— -When 

 a biologist, especially a zoogeographer, undertakes to 

 deal with fossils, he becomes of necessity somewhat of 

 an iconoclast. The faunas of the several horizons 

 represent to him not so many distinct and separate suc- 

 cessive faunas each derived directly from that preceding, 

 but so many distinct faunal regions, each equidistant 



