156 HYATT, 



gizes the parietal muscles of Paludicella with the parietal 

 muscles of Lingula. 



The parietal muscles of the former genus are merely 

 local developments of the transverse muscular fibres of 

 the third layer similar to the less prominent bands occa- 

 sioning the annular folds in the ccencecium of Pluma- 

 tella (PI. 8, fig. 10). Mr. Hancock, although he has 

 pointed out the identity of the adjusters of the Brachio- 

 pod with the so-called opercular muscles (retentors) of 

 Bowerbankia and Paludicella, denies the existence of the 

 opercular muscles in Fredericella. He describes and 

 figures the retentors, but does not consider them, al- 

 though similarly situated in the neighborhood of the 

 orifice, to be the equivalents of the opercular muscles of 

 Bowerbankia and Paludicella on account of their great 

 functional divergence. I have failed to find any great 

 functional divergence between the muscles of the orifice 

 in Paludicella and Fredericella. The only difference 

 between them appears to be, that in Paludicella they are 

 fewer in number and retain two folds of the endocyst 

 from complete evagination instead of one, and in Bower- 

 bankia they do not retain any fold, only becoming active 

 when the polypide is invaginated. They perform nearly 

 about the same function in all these genera, the invasrina- 



o 7 o 



tion of the lower part of the evaginable endocyst, the 

 differences are those of degree only. 



Facts of position, being always of determinate value, 

 are more reliable than functional resemblances, however 

 close they may be ; for although the latter often afford 

 a clue to the true homology, they furnish, in the present 

 state of our knowledge, but a very precarious means for 

 estimating the degree of similarity between the parts of 

 different animals. Functionally, for example, the mus- 

 cles of an Avicularian are as widely separable from those 

 of the normal forms on the same stock, as the occlusors 

 and divaricators of the Brachiopod from the retractors of 

 the Polyzooid. They open and close the valv se of a 

 shell instead of retracting a polypide. Such an extrava- 

 gant divergence between identical organs in two zooids 

 of the same compound form is decisive against the adop- 



