78 NEW YORK STATE MUSEUM 



expansion of their surface and the increased friction in sinking. 

 The single dorsal spine of Cyphaspis is, for example well comparable 

 in function to the floating spine of the zoaea of crustaceans. Taking 

 the case of the three species of Triarthrus, associated in the Glouces- 

 ter shale; they should have had according to biologic principles 

 different habits of life to avoid the disastrous competition among 

 closely related forms ; and it is very probable that the smooth T . 

 glaber and eatoni beckiauct.) were more active bottom- 

 swimmers, while the long-spined T . s p i n o s u s was more given 

 to a floating habit in a little higher level of the water. 



There remain the peculiar cases of reversions noted by Doctor 

 Clarke in Devonian trilobites of South America and by Doctor 

 Walcott in Bronte us. Hydrocephalus and the Proparia ; 

 and expressed in the intergenal spines at the base of the head and 

 in the lateral spines of the free cheeks. It seems impossible to con- 

 sider these cases of reversion as due to arrested development at an 

 immature stage, for Beecher's studies of the protaspis-stages of vari- 

 ous post-Cambrian genera have failed to reveal these archaic lateral 

 and intergenal spines in the ontogenetic development of the later 

 genera. Nor are they cases of meristic variation, especially not the 

 intergenal spines. Thcie would thus remain as possible explanation 

 for them the assumption that they also present latent characters 

 never actually lost but only submerged for longer or shorter time by 

 the interpolation of inhibiting factors. 



Bibliography 



Billings, E. Geology of Canada, 1863, p. 202, fig. 199 

 Barrande, J. Sysemc Silurian de la Boheme, v. i, 1852, sup. 1S72 

 Walcott, C. D. Second Contribution to the Studies of the Cambrian 



Faunas of North America. U. S. Geol. Survey; Bui. 30, 1886 

 Clarke, J. M. Fossei's Devonianos do Parana. Monographias do Service 



geologico e mineralogico do Brasil, 1913 

 Hall, J. & Clarke, J. M. Paleontology of New York. v. 7, 1888 

 Walcott, C. D. Olenellus and Other Genera of the Mesonacidae. Smiths. 



Misc. Coll. V. 53, p. 229. 1910 

 Grabau, A. W. Principles of Stratigraphy. New York, 1913 

 Walcott, C. D. Cambrian Geology and Paleontology. III. No. 5. Cam- 

 brian Trilobites. Smiths. Misc. Coll. v. 64, p. 301. 1916 

 Arber, A. On Atavism and the Law of Irreversibility. Amer. Jour. Sci., 



48 :27. 1919 

 Lull, R. L. Organic Evolution. New York, 1917 



Richter, R. & E. Von unseren Trilobiten. II. 47. Ber. Senckenbergische 

 Naturf. Gesellsch. 1918 



