604 THE SPIRAL ^SHELLS [ch. 



whether the old one be in the form of a single, solitary chamber, 

 or have already attained to the form of a chambered or spiral 

 shell. This is precisely what often happens in the case of Orbuhna, 

 when within the spherical shell we find a small, but perfectly 

 formed, spiral "Globigerina*." 



The various Miholidae (Fig. 311), only differ from the typical 

 spiral, or rotaline forms, in the large angle subtended by each 

 chamber, and the consequent abruptness of their inchnation to 

 each other. In these cases the outward appearance of a spiral 

 tends to be lost ; and it behoves us to recollect, all the more, 

 that our spiral curve is not necessarily identical with the outline 



A B 



Fig. 317. A, Tertularia trochus, d'Orb. B, T. concava, Karrer. 



of the shell, but is always a line drawn through corresponding 

 points in the successive chambers of the latter. 



We reach a limiting case of the logarithmic spiral when the 

 chambers are arranged in a straight line; and the eye will tend 

 to associate with this hmiting case the much more numerous forms 

 in which the spiral angle is small, and the shell only exhibits a 

 gentle curve with no succession of enveloping whorls. This 

 constitutes the Nodosarian type (Fig. 87, p. 262) ; and here again, 

 we must postulate some force which has tended to keep the 

 chambers in a rectihnear series : such for instance as gravity, 

 acting on a system of "hanging drops." 



* Cf. Schacko, G., Ueber Globigerina -Einsohluss bei "Orbiilina, Wiegmaniv's 

 Archio, XLix, p. 428, 1883; Brady, Chall. Rep., p. 607, 1884. 



