CLASSIFICATION CAMBRIAN BRACHIOPODA — WALCOTT 1 53 



the corneous shells of the Atremata and Neotremata, and, as far as 

 known to me, all of the Cambrian corneous shells are of this type. 



The figures on the accompanying plate, with the exception of 

 figures II and 12, are from photographs which have not been re- 

 touched. Unfortunately higher magnifications could not be used 

 without a loss of clearness ; but, even at the present magnification, 

 these views show a decided difference in structure. 



In conclusion, it appears that the Cambrian Billingsellidae are 

 further removed from the Ordovician and later Protremata than 

 hitherto suspected, the microscopic shell structure in the former 

 being of granular material pierced by small pores and in the latter of 

 fibrous material. On the other hand, the microscopic structure of 

 the Cambrian and later Pentameracea is so similar that an unbroken 

 line of descent is indicated. 



TERMINOLOGY RELATING TO THE SHELL 



The definitions given in the following pages are largely those of 

 Schuchert [1897, pp. 73-75], with the exception of the muscle scars 

 of the inarticulate brachiopods. For the Atremata and Neotremata 

 the terminology proposed by Professor William King [1873, pp. 5, 6] 

 is adopted, and for the Protremata that used by Messrs. Hall and 

 Clarke [1892, pp. 183-188] and given under the terminology of 

 Schuchert [1897, pp. 73-77]. I agree with Messrs. Hall and Clarke 

 that Professor King's terminology has claims for its adoption, owing 

 to its simplicity. Dr. F. Blochmann has proposed [1900, p. 108] a 

 set of terms for the muscles of the inarticulate brachiopods that has 

 much to commend it. The terminology of Mr. Albany Hancock 

 [1859, p. 800] has been extensively used by authors. The numbers 

 below correspond to the numbers given the terminology of King, 

 Schuchert, and Blochmann. 



Hancock, 1859 

 Inarticulates Articulates 



1. Anterior occlusors. i. Anterior occlusors. 



2. Posterior occlusors. 2. Posterior occlusors. 



3. Divaricator. 3. Accessory divaricators. 



4. Central adjusters. 4- ) ir 1 i- , 



T^ ^ , J. ^ y Ventral adjusters. 



5. External adjusters. 5- j , 



6. Posterior adjusters. 6. Dorsal adjusters. 



7. Peduncular. 7. Peduncular. 



