CLASSIFICATION CAMBRIAN BRACHIOPODA — WALCOTT I49 



STRUCTURE OF THE SHELL 



The classification of the Protrematons genera is so profoundly 

 influenced by the structure of the shell that it was decided to include 

 the following .notes : 



The general structural characters of the shell of the Ordovician 

 and later brachiopoda have been so fully described by authors that 

 it does not appear to be necessary or desirable to repeat them. The 

 student will find a full description given by Messrs. Hall and Clarke 

 in their "Introduction to the Study of the Brachiopoda" [1892, pp. 

 150-225]. 



Some of the more important works that contain data on the struc- 

 ture of the shell are Hancock, "On the Organization of the Brachio- 

 poda" [1859, pp. 791-869] ; King, "On Some Characters of IJngiila 

 anatina" [1873, pp. 1-17] ; Carpenter, "On the Intimate Structure of 

 the Shells of Brachiopods" [1853, pp. 23-45] ; Davidson, "On the 

 Classification of the Brachiopoda" [1853, pp. 41-136] ; and Mickwitz, 

 "Ueber die Brachiopodengattung Oholus" [1896]. 



The greater proportion of the Cambrian brachiopoda is largely 

 corneous or chitinous. These brachiopoda are restricted to the in- 

 articulates, but the inarticulates of the Cambrian do not all possess 

 corneous shells. Dr. Mickwitz has shown [1896, pp. 102-142] that 

 the shells of Obolus and its subgenera are essentially the same as 

 those of Lingula in composition and structure. In both the shells 

 are composed of successive calcareous and corneous lamellae that 

 vary in thickness and structure. The calcareous lamellae are pris- 

 matic and penetrated by minute tubules ; the corneous lamellae are 

 compact and imperforate. 



Messrs. Hall and Clarke, in speaking of the shells of the articu- 

 late brachiopoda, say: "Among the articulate genera, under favor- 

 able preservation, there may be distinguished three distinct calcareous 

 shell layers : an inner prismatic or fibrous layer, which constitutes 

 the greater portion of the shell ; above this is a thin lamellar layer, 

 and the outer surface of the shell is covered by a tenuous epidermal 

 film or periostracum. When the shell is punctate the tubules open 

 on the inner surface in narrow apertures, whence they widen up- 

 wards, abruptly expanding in the lamellar layer, at whose upper 

 margin they terminate. They do not pierce the periostracum." 

 [1892, p. 175.] 



Among the calcareous, inarticulate brachiopoda the shell of the 

 Cambrian genus Oholella shows a dense, compact, slightly lamellated 

 structure made up of a granular ground-mass pierced by extremely 

 small tubules or pores. The substance of the shell of Rusfella and 



