CLASSIFICATION CAMBRIAN HRACIIIOPODA WALCOTT I 59 



SpondyIvIUM. — A plate in some articulate brachiopoda, mainly the 

 Pentameracea, formed by the imion of converging dental plates, to 

 the upper surface of which are attached the adductor, diductor, and 

 pedicle muscles. The spondylium may rest upon the ventral valve 

 or may be supported by a median septum. The spondylium appears 

 to be first indicated in the articulates by a thickening of the shell of 

 the ventral valve beneath the umbonal region so as to form an area 

 upon which all the muscles of the valve have their points of attach- 

 ment. In BiUingscUa this is beautifully illustrated by B. cxporccta 

 and B. pUcatclla. In its development the spondylium is foreshad- 

 owed in the Atremata by the so-called platform of Fordinia and the 

 still more primitive form in O bolus. For the purpose of reference, 

 the rudimentary spondylia attached directly to the inner surface of 

 the valve, as in BiUingscUa, may be called pseudospondylia (sessile 

 spondylia, Ulrich), and those free or supported by a septum or 

 septa, spondylia. In the Cambrian Atremata the homologous equiv- 

 alent has been known as the platform. In Oholus, etc., there is 

 sometimes developed in the dorsal valve a plate similar in appear- 

 ance to the spondylium, but different in origin and known as the 

 cruralium. 



Teeth. — Two processes of the ventral valve of articulate brachio- 

 poda, serving for articulation. 



Transmedian (Rotator) Muscles. — In Obolacea these are sit- 

 uated posteriorly just in advance of the umbonal muscle, two on one 

 side and one on the other. By their contraction the dorsal valve 

 turns alternately first in one direction and then in the other, 



Traxsverse Axis. — A line through the shell from right to left, 

 midway between the beak and anterior region. (See Longitudinal 

 axis.) 



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 are attached. 



Umbo.— The elevated or prominent portion of the valve anterior 

 to the apex. 



Umbonal 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. 



Umbonal Slopes. — The inclined surfaces about the umbo and 

 opposite the cardinal slopes. 



\'Entral Valve. — Usually the larger valve situated on the ventral 

 side of the animal. Among articulate brachiopoda the valve is usu- 



