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NEW YORK STATE MUSEUM 



describe these cephalopods as larger and the aperture of the shell as 

 wider in the male than in the female. 



Emphasizing the general observation of the larger size of the 

 females among the cephalopods, we incline to accept Dean's view and 

 especially do not believe that the doubt still expressed as to the 

 relative size of the shells in Nautilus militates against our conclusion 

 that the two different sizes of the mature conchs in Oncoceras 

 pupaeforme can be ascribed to sexual differences and that 

 the larger form most probably represents the female. D'Orbigny, 

 noticing that there were two varieties of almost every kind of am- 

 monite, one compressed, the other inflated, assumed that the first 

 were the shells of male individuals, the second of females (see 

 Woodward, 1910, p. 185), but Buckman and Bather (1894, p. 427) 

 have concluded that the sexual differences in ammonites, inferred 

 by de Blainville D'Orbigny, Munier — Chalmas and Haug are 

 "auxologic or bioplastic rather than sexual, being in some cases 

 phylogerontic, in others merely ephebic or gerontic." 



In regard to possible sexual differentiation of the shells in the 

 Paleozoic forms, especially the early orthoceraconic nautiloids, we 

 have not been able to find any suggestions in the literature at our 

 disposal, which, however, is not complete except in Barrande's 

 Syste^ne Siluricn (v. 2, pt i, p. 38 ff, 1877). There the fact of the 

 wide differences in the relation of length of living chamber to the 

 width at the base of the same in the mature stage of a number of 

 species is shown in tables; this difference rising in some species, as 

 Orthoceras cu Iter, to a threefold length of the chamber in 

 the longer forms. While Barrande would see in these wide differ- 

 ences of mature development of the living chamber only individual 

 variations in size, (loc. cit. p. 39), we consider it very probable 

 that they also denote the sexual difference in the size of the shell 

 in these species. 



Bibliography 



Buckman & Bather. Natural Science, i8g4, 4:427 



Lankester, E. Ray. A Treatise on Zoology. Pt 5, Mollusca by Paul Pel- 



seneer. London, 1906 

 Dean, Bashford. American Naturalist, 1901, 35 :8r9 

 Woodward, S. P. A Manual of the Mollusca, London, 1910 



3 On Some Cases of Reversion in Trilobites 



T r i a r t h r u s s p i n o 3 u s was described by Billings (1857, 

 p. 340) as characterized by a long spine that springs from the neck 

 segment and a second one proceeding from the eighth segment of 

 the thorax ; and two long genal spines. 



