270 C. P. Reyes — Sedentary habits of Platyceras. 



horizons ; and this has given rise to the establishment of many 

 species which are nn questionably invalid. The synonymy of 

 the species of Platyceras when fully worked out will doubt- 

 less result in quite an extensive numerical reduction of the 

 species. In the absence of better defined characters for specific 

 distinction considerable importance has been attached to the 

 configuration of the peristome, but even this character in the 

 majority of the species of this genus now appears to have little, 

 if any, classificatory value. A careful comparison of a large 

 series of different species of Platyceras reveals the fact that 

 the apertural margin in various specimens of the same species 

 often presents considerable variation : a phenomenon not to be 

 entirely unexpected in a group so closely allied to the modern 

 Caspulus, from which some writers even now question the pro- 

 priety of generically separating Platyceras. Among the living 

 Calyptrceidoe it has been observed that both color and form are 

 to a great extent dependent upon individual environment, and 

 hence among forms of the same species there may be many 

 varietal phases. It has further been noted that the majority 

 of the members of this family attach themselves, while yet 

 quite young, to stones and shells of other Mollusca and, having 

 once found a suitable situation, seldom, if ever, remove from 

 the spot where they first settled. The character and contour 

 of the surface on which they have settled would therefore 

 determine the form and outline of the apertural margin. The 

 sedentary habits of the modern representatives of this group of 

 Mollusca would be suggestive of analogous habits among their 

 fossil congeners. Notwithstanding the comparative abundance 

 of Platyceras in some of the Paleozoic strata of both this 

 country and Europe direct paleontological evidence of the 

 sedentary habits of this group is not often met with ; yet the 

 instances presented, independent of their bearing upon Platy- 

 ceras, are of unusual significance as furnishing a solution to 

 certain important morphological problems relative to the pale- 

 ozoic crinoids. From time to time paleontologists have men- 

 tioned the occurrence of Platyceras attached to crinoids, and 

 numerous explanations have been advanced, but it was not 

 until 1873* that the probably correct solution was given. A 

 discussion of the various explanations offered prior to this date 

 is not necessary since some of them have been fully considered 

 by Meek and "Worthen,f who have also pointed out clearly the 

 improbability of the claims of the Austins;); in their elaborate 

 and highly imaginary account, and others, that the crinoids 

 perished while in the act of devouring the Platyceras. Not 

 only is this view highly improbable but its absurdity is only 

 too manifest when the character of the food of both these 



* Geol. 111., vol. v, p. 334 et seq. f Loc. cit. % Recent and Fossil Crinoidea. 



