786 THE EQUIANGULAR SPIRAL [ch. 



magnitudinem adjacentis plani; sed et, ipsius ope, Limacum et 

 Conchiliorum domunculos metitur. Existimat utique, magna veri- 

 similitudine, domunculos hosce non alios esse quam Pjrramides 

 convolutas: quarum Axis sit, istiusmodo Spiralis: non quidem in 

 piano jacens, sed sensim in convolutione (circa erectum axim) 

 assurgens: pro variis autem curvae, sive ad rectam circumductam 

 sive ad subjacens planum, " angulis, variae Conchiliorum formae 

 enascantur. Atque hac hjrpothesi, mensurata Pyramide, metitur 

 etiam ea conchiliorum spatia." 



For some years after the appearance of Moseley's paper, a number 

 of writers followed in his footsteps, and attempted in various ways 

 to put his conclusions to practical use. For instance, d'Orbigny 



Fig. 375. d'Orbigny's helicometer. 



devised a very simple protractor, which he called a Helicometer*, 

 and which is represented in Fig. 375. By means of this little 

 instrument the apical angle of the turbinate shell was immediately 

 read off, and could then be used as a specific and diagnostic character. 

 By keeping one limb of the protractor parallel to the side of the 

 cone wjiile the other was brought into hne with the suture between 

 two adjacent whorls, another specific angle, the '^sutural angle," 

 could in like manner be recorded. And, by the hnear scale upon 

 the instrument, the relative breadths of the consecutive whorls, 

 and that of the terminal chamber to the rest of the shell, might 



* Alcide d'Orbigny, Bull, de la*soc. geol. Fr. xiii, p. 200, 1842; Cours elem. 

 de PaUontologie, ii, p. 5, 1851. A somewhat similar instrument was described by 

 Boubee, in Bull. soc. geol. i, p. 232, 1831. Naumann's conchyliometer {Poggend, 

 Ann. Liv, p. 544, 1845) was an application of the screw-micrometer; it was provided 

 also with a rotating stage for angular measurement. It was adapted for the 

 study of a discoid or ammonitoid shell, while d'Orbigny 's instrument was meant 

 for the study of a turbinate shell. 



