330 
The American Geologist. 
May, 1894 
tubes (Comulites, ConchicoHtes,Serpufct; etc.); and the obser- 
vations by Jackson* upon the effects of attachment in the pe- 
lecyppds would lead to the inference, home out by the evi- 
dence afforded by the brachiopods and such annelids as these, 
that such shell-structure is not merely a concomitant of this 
solid fixation, but a result of it. 
Figs. 4, 5, 6. Lateral, apical and umbilical views of Autodetus lind.ttroemi. x'.l. Ham- 
ilton shales, Canandaigua lake, N. Y. 
It will be further observed that the whorls in these shells 
are but slightly oblique, and in completed individuals these 
appear to have numbered four and one-half or five. Some of 
the specimens of A . beecheri, preserved as external and par- 
tial internal casts, afford facts bearing upon the mode of early 
growth of the shell. Figure 7 represents a very young shell, 
which is in no respect unlike forms of Spirorbis at a corres- 
ponding growth stage. The nuclear vnd is very obtuse, the 
surface of the primary whorl becoming abruptly and ex- 
tremely convex, while the aperture is depressed and its outer 
margin greatly produced. At this stage and for a consider- 
able subsequent period in the growth of the shell there is no 
envelopment of the whorls, the mode of growth being in all 
respects like that of Spirorbis, fixation being effected only by 
the attachment of the lower surface of the whorls. This is 
seen in figure 9, a view of the internal 
cavity of the cone in which an internal 
cast of the early whorls is retained. With 
the formation of the volution next suc- 
ceeding those represented by this cast, 
the plane of the volution becomes eleva- 
ted above that of the earlier whorls and 5fti d^KSrif h |" 
the conical form of the mature shell has LowerOr«kan y ,Becraft' B 
*Phylogenj of the Pelecypoda; Mem. Boston Society Natural History, 
vol. iv. p. 322, 1890. 
