IO4 NEW YORK STATE MUSEUM 



Observations. In describing this species, Professor Hall stated that it 

 differed from T. desplainense in its more numerous and more sharply 

 elevated annulations, which do not increase beyond the point opposite the 

 apparent apex of the shell. As " a conspicuously distinguishing feature," 

 is cited the sinistral direction of the volutions. A comparison of the speci- 

 mens here described with our material of T. desplainense leaves no 

 doubt as to the greater prominence and closer arrangement of the costae 

 in that species, characters which give a distinctly different habit to the 

 shell. In regard to the sinistral enrolment, Hall deemed it necessary to 

 add an explanatory note, stating that, as inT. desplainense the inner 

 volutions are sometimes a little depressed below the outer, it might perhaps 

 be suspected that T. costatum is an exaggerated condition of the for- 

 mer species, with the inner volutions still more depressed. This, how- 

 ever, is claimed not to be the case, as T. costatum is clearly sinistral. 

 It is added that Barrande described several sinistral forms of Trochoceras 

 and found that "the enrolment is sometimes dextral and sometimes sinis- 

 tral according to the species, but the dextral forms greatly predominate." 

 In two forms, however, viz T. asperum and T. sandbergeri, Bar- 

 rande concedes that "we find both modes, varying in individuals." Bar- 

 rande's descriptions and fine figures of these two species, show that they are 

 but very slightly asymmetric. They belong therefore near the beginning 

 of the morphologic series, which according to Barrande, extends from 

 perfectly symmetric to the highly asymmetric torticones ; and it may be 

 inferred that the tendency to become asymmetrically enrolled to the right 

 side had not yet become established in these Bohemian forms. The two 

 sinistral species described by Hall, viz T. costatum and T. aeneas, 

 when compared with the decidedly dextral T. desplainense, are con- 

 spicuously less asymmetric, and therefore appear to represent an earlier and 

 less fixed stage. We are disposed to believe that the species of Trocho- 

 ceras can not be naturally divided into a sinistral and a dextral series, but 

 that, as by far the prevailing number of species and those the most pro- 

 gressed, are dextral and the sinistral are but slightly asymmetric, the gen- 



