Report of the State Geologist. 



457 



especially those of the genera Strophomena, Rapinebqi ina, :: Stro- 



PHEODONTA, Lkit i;.NA, pLEOTAMBONITKfl, ChONETES, PrODU< lis. fete. 



Fvxation by calcareous cementation also began early in life, and 

 in Crania, Davidsonia, Lept enisoa and Riohthofenia, continues 

 during the remainder of the animal's existence, the attached 

 valve frequently being fixed by its entire outer surface. In other 

 genera the condition of attachment was restricted to early, or 

 adolescent growth stages ; (Orthothetes, Derijya, Strophalosia, 

 Oldbamina, etc.),t the shell in after life becoming detached from 

 its mooring, but usually retaining a scar or deformation of the 

 surface in the umbonal region. Attachment by cementation has 

 been shown by Jackson to induce among the Lamellibranchiates 

 a strongly vescicular structure in the attached shell ; and it may 



Fig. 3.— A fragment of a erinoid column cov- Fig. 4.— Lower valves of Crania quadrat a 



ered with a number of individuals of Crania attached to column of a erinoid, the upper 



socialis. which retain the marks of the valves having been removed by natural 



sutures between the segments of the erinoid. causes (Davidson). 



be a rule of more general application, as it is seen in certain 

 attached annelidan tubes, e. g., Comulites proprius ; and among 

 those brachiopods which are attached by a considerable surface 

 of the shell, such as Crania and, especially, the extravagant 

 Kichthofenia, the vesciculation of the shell-structure is a striking 

 feature. 



* Not infrequently in Strophomena and Rafinesquina a minute pedicle-passage is retained in 

 the mature condition of the shell, but it is always evident that the shell could not, at this 

 period of growth, have been supported by any muscular apparatus which might be protruded 

 through this aperture. In Rafinesquina altemata, of the Lower Silurian, it is seen that the 

 contraction of the aperture has increased most rapidly about the base of the pedicle, and the 

 internal opening of the passage may be altogether closed, while the external opening remains. 



t Craniella Ulricki, of the Lower Silurian, is the only inarticulate brachiopod which appears 

 to have been thus attached in early growth and to have become free at maturity. 



1891. 58 9 



