428 NATURAL SCIENCE. June, 



doubt, among Ammonites, the compressed forms are almost always 

 accompanied by swollen forms, and that these two forms are con- 

 stantly associated. The ornamentation and septa are identical ; the 

 individuals only differ in the involution and width of their whorls. 

 Anyone making a general study of Ammonites can easily convince 

 himself that the majority of species have two distinct forms, and can 

 prove this whenever the material is sufficiently plentiful. To what 

 cause can this difference of thickness be attributed if not to sex ? 

 Ornamejnt is often more pronounced in one sex than in the other, 

 although identical in general arrangement. The ribs and tubercles 

 are then more marked and their number is less ; in this case the 

 furrows have a greater depth {A . bifrons, Muychisone, Martiusii [szV] , 

 Tniellei, Duncani, etc.). In other species the ornament is in no way 

 changed, the whorls preserve their ordinary proportions, and are not 

 modified in any of their parts." 



That differences, not of specific importance, do exist between 

 individual shells, will not be denied by anyone who has studied large 

 collections of Ammonoidea ; and these differences may be due to sex, with 

 at least as much probability as to any other cause. The smaller size of 

 the male is certainly paralleled among recent Cephalopoda, e.g., Sepia, 

 Sepiola, Loligo, Argonauta. For the views enunciated by Reynes and 

 the other palaeontologists mentioned above, something might perhaps 

 be said within small limits ; but modern research has done much to 

 disprove them. First, the supposed males and females are found to 

 be not always contemporaneous when the strata are critically 

 examined. Next, the thick and thin forms of a " species " are re- 

 placed in modern collections by a great number of forms in all 

 degrees of compression, and these forms do not exhibit that strict 

 agreement in details of ornamentation, etc., which we should expect 

 to find after reading the accounts of these authors. Finally, it has 

 yet to be proved that the thicker forms did actually possess body- 

 chambers of larger capacity than their thinner relatives. It would be 

 so, of course, if the length remained the same ; but our investigations 

 rather indicate that the length varies with the degree of compression, 

 so as to maintain, among relatives, the same size of body-chamber 

 in proportion to the whole shell. For instance, in Stephanoceras 

 the body-chamber varies in length from about half a whorl in 

 the thick forms (the supposed females) to very nearly two whorls in 

 the thin forms (the supposed males). If the " female " had to enlarge 

 her body-chamber for the accommodation of the eggs, it is obvious 

 that her body-chamber ought to be larger in proportion to the whole 

 shell than is the body-chamber of the " male," who was not called 

 •upon to provide this extra house-room. 



Recent writers on the subject have therefore gone upon another 

 tack. In their over-anxiety to obey the behest " cherchez la femme ! " 

 two well-known French authors have set themselves to prove that the 

 two sexes in the Ammonites differ not only in compression, but in 



