so conspicuous in the {568} majority of univalves, or to let us measure or estimate the magnitude of the apical angle (ö¡) of the enveloping cone. This however we can do in forms like Isocardia and Diceras; while in Caprinella we see that the whorls lie in a plane perpendicular to the axis, forming a discoidal spire. As in the latter shell, so also universally among the Brachiopods, there is no lateral asymmetry in the plane of the generating curve such as to lead to the development of a helix; but in the majority of the Lamellibranchiata it is obvious, from the obliquity of the lines of growth, that the angle ö¡ is significant in amount.
The so-called ãspiral armsã of Spirifer and many other Brachiopods are not difficult to explain. They begin as a single structure, in the form
of a loop of shelly substance, attached to the dorsal valve of the shell, in the