1897.] NEW YORK ACADEMY OF SCIEKCKS. 79 



tween the marginal lines of the Saxidomus. Moreover, all other 

 specimens of the Anomiidai examined (which include about 

 three dozen individuals) show absolutel}^ no trace of similar 

 radial markings on any portion of the shell. These facts lead 

 me to believe that the lines in question are really assumed from 

 a surface of attachment. The inner umbonal region is quite 

 smooth, and represents the youngest period of the shell when 

 probably it was quite free. 



From the extent of the adaptation it is evident that the Jrlacua- 

 nomia became ver}' early attached to the Saxidomus. Its extreme 

 marginal position would seem to indicate that the attachment 

 did not occur until the Saxidomus had very nearly, if not entirel}', 

 completed its growth. 



Among the Anomioid shells collected by the expedition there 

 is one specimen that shows an assumption of low ridges or folds 

 instead of sharp lines, but unfortunately the attached surface 

 was not preserved. Still other specimens do not show assumed 

 lines, although attached to more or less rigid shells. 



The phenomenon here presented b}^ the Grepidula (Graf.) and 

 the Anomioid shells is not new ; it has been frequently ob- 

 served and commented upon by paheontologists in fossil shells, 

 and raises interesting questions as to the specific valne of sur- 

 face configuration in attached forms. Thus Keyes* calls atten- 

 tion to the fact that in Platyceras, a Gasteropod found attached 

 on the vault of Palseocrinoids, " the longitudinal folds and plica- 

 tions in the shell are in many cases due largeh^ to the surface of 

 attachment," and that even certain nodes on the shell " appear 

 to have resulted from the peculiar nodose ornamentation of 

 Platycrinus.''^ He concludes, therefore, that, as the configuration 

 of both the surface and the peristome in these attached shells 

 (including also the Calyptrjeidae) " being to a great extent de- 

 pendent upon individual environment," but little specific value 

 is to be attached to them. Barrandef figui'es a " Discina ? 

 miranda " with folds that must have resulted from the surface 

 of attachment. HallJ figures with brief comments half a dozen 

 species of Crania that attach themselves to various surfaces, 

 such as other Brachiopod and Gasteropod shells, crinoid stems, 

 etc., and each in case the Crania copies accurately the surface 

 configuration. An instance of this is seen in the Geological 

 Museum of the School of Mines, which contains a Crania 

 scabiosa attached to a Strophomena and minutely copying the 

 excessively fine radial striations of the latter, 



*" The Sedentary Habits of Platvceras," Am. Jour. ofSci., Oct., 18S8;and "On the 

 Attachment of Platyceras to Palabcrinoids and its effects in modilying the forms of 

 the Shell," Proc. Am. Ph. Soc, Vol. XXV., No. 128, 1888. 



tSysteme Silurien de Boheme, Vol. V. pi. 129. 



fPal. N. Y., Vol. VIII. Brachiopoda, I., pi. iv., H. 



