XI] 



OF THE MOLLUSCAN SHELL 



529 



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 

 devised a very simple protractor, which 

 he called a Helicometer*, and which 

 is represented in Fig. 267. By means 

 of this little instrument, the apical 

 angle of the turbinate shell was im- 

 mediately 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 while the other was brought 

 into line with the suture between two 

 adjacent whorls, another specific angle, 

 the "sutural angle," could in like 

 manner be recorded. And, by the 

 linear scale upon the instrument, the 

 relative breadths of the consecutive 

 whorls, and that of the terminal 

 chamber to the rest of the shell, 

 might also, though somewhat roughly, 

 be determined. For instance, in 

 Terebra dimidiata, the apical angle 

 was found to be 13°, the sutural 

 angle 109°, and so forth. 



It was at once obvious that, in 

 such a shell as is represented in 

 Fig. 267 the entire outline of the 

 shell (always excepting that of the immediate neighbourhood of 



* Alcide D'Orbigny, Bull, de la soc. geol. Fr. xin, p. 200, 1842; Cours dem. 

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

 Boubee. in Bvll. soc. geol. i, p. 232, 1831. Naumann's Conchyliometer {Poggevd. 

 Anil. 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 

 ^udy of a discoid or ammonitoid shell, while D'Orbigny's instrument was meant 

 for the study of a turbinate shell. 



T r- 34 



Fig. 267. D'Orbigny's 

 Helicometer. 



