296 . CAMBEIAN BKACHIOPODA. 



Trapezoidal area. — The area on each side of the heart-shaped cavity in Oholus in which the 

 outside and middle lateral scars and central muscle scars were attached. (See PI. VII, figs. 

 10 and 12.) 



TJmho. — The elevated, or prominent portion of the valve anterior to the apex. 



TJmbonal cavity. — The hollow space in the interior of the shell beneath the umbo. 



Umbonal muscle. — A single muscle situated in the umbonal region of most Atremata. By- 

 its contraction the valves are opened anteriorly. In Oholus this muscle divides toward the 

 ventral valve. 



TJmbonal slopes. — The inclined surfaces about the umbo and opposite the cardinal slopes. 



Vascular (pallial) sinuses. — Two convergent or divergent primary sinuses of the circulatory 

 system, traversing the mantle and originating in the posterior medial region. They usually 

 have numerous secondary (lateral and peripheral) branches, and both often leave impressions 

 in the shell. (See p. 311.) 



Ventral valve. — Usually the larger valve situated on the ventral side of the animal. 

 Among articulate brachiopods the valve is usually easily distinguished by the presence of a' 

 delthyrium or pedicle opening through which the pedicle is protruded. In man}^ atrematous 

 genera the ventral valve is not readily distinguished. When the shell is cemented to foreign 

 bodies it is always by the ventral valve. It is usually the larger and deeper of the two valves. 

 Pedicle, larger, dental, neural, and receiving valves are synonymous terms. 



Visceral area. — The posterior region of the interior of the valves between the pallial sinuses ; 

 in general the immediate area of the median muscle tracks. 



Visceral cavity. — Visceral area. 



THE BEACHIOPOD SHELL. 



STRUCTURE. 



GENERAL, STATEMENT. 



The general structural characters of the shell of the Ordovician and later brachiopods 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 Hall and Clarke in their "Introduction 

 to the study of the Brachiopoda" [1892a, pp. 150-225]. 



Some of the more important works that contain data on the structure of the shell are those 

 of Hancock," King,* Carpenter,"" Davidson,'' and IMickwitz.' 



The greater proportion of the Cambrian brachiopods are largely corneous or chitinous. 

 These are restricted to the inarticulates, but the inarticulates of the Cambrian do not all pos- 

 sess corneous shells. Mickwitz has shown [1896, pp. 102-142] that the shells of Oholus 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 lamella; are prismatic and crossed by minute tubules; the 

 corneous lamellae are compact and imperforate.- 



Hall and Clarke [1892a, p. 175], in speaking of the shells of the articulate brachiopods, say: 



Among the articulate genera, under favorable 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 narro-w apertures, -whence they widen up-ward, abruptly 

 expanding in the lamellar layer, at -whose upper margin they terminate. They do not pierce the periostracum. 



a Hancock, A., On the organization of the Brachiopoda, 1859, pp. 791-869. 



b King, W., On some characters of Liti^ula anatina, 1873, pp. 1-17. 



^- Carpenter, W. B., On the intimate structure of the shells of Brachiopoda, 1853, pp. 23-45. 



"i Davidson, T., On the classiflcation of the Brachiopoda, 1853, pp. 41-136. 



« Mickwitz, A., Ueber die Brachiopodengattung Oholus, 1896. 



