76 SYNOPSIS OF AMERICAN FOSSIL BKACIIIOPODA. [bull. 87. 



Neotreniata, and Piotreniata, and subsequently becoming more or less 

 firmly attached to either the dorsal (Atremata) or ventral valve. 



Protef/ulum ( Beecher). — The initial shell of brachiopods. It is smooth 

 and of microscopic size, in outline bein,n- semicircular or arcuate, and 

 without cardinal areas. 



Protractor muscles. — In the Lingulacea one pair has the ventral ends 

 fastened at the anterior extremity of the visceral area, extending back- 

 ward and inserted near the lateral margin of the dorsal valve, outside 

 the rotators. A second ])air originates just behind the adductors of 

 the ventral valve, and is inserted posterior to the first pair. These 

 muscles draw the dorsal valve forward. They are apparently present 

 in the Obolidie and Trimcrellid;e, but their position is different. 



Pseudodeltiduim. — Properly this term applies only to the united con- 

 dition of the deltidid plates in the Protremata and Telotremata. It 

 is provisionally applied to the concave or convex medial portion of the 

 cardinal areas in Atremata and Protremata. 



Retractor muscles. — In the Atremata these extend from the outer 

 lateral margins of the visceral area in the ventral valve to its anterior 

 extremity in the dorsal valve, and serve to readjust the dorsal shell. 



Rotator muscles. — In Lingulacea these are situated 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. 



Septal 2)lates. — Plates supporting the crural processes, also known as 

 crural 2^1 tttes. 



Spondylium. — A plate in the Pentameracea, formed by the union 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. 

 This plate is rarely present in the Telotremata, but more commonly 

 in the Atremata, where it is known as the platform. There is some- 

 times developed in the dorsal valve a plate similar in appearance to 

 the spondylium, but different in origin, and known as the cruralium. 



Spiralia (Beecher). — The calcareous sjiiral brachial supports in the 

 Spiriferacea. A connecting jugum may be present or absent. 



Syrinx. — A tubular structure developed in the delthyrium of some 

 Spiriferacea, opening ventrally and partially inclosing the pedicle. 



Teeth. — Two j^rocesses of the ventral valve of articulate brachiopods, 

 serving for articulation. 



Transverse axis. — A line through the shell from right to left, midway 

 between the beak and anterior margin. 



Umbo. — The elevated or i^rominent portion of the valve anterior to 

 the apex. 



Umbonal muscle. — A single muscle situated in the umbonal region of 

 most Atremata. By its contraction the valves are opened anteriorly. 

 In Obolus this muscle divides toward the ventral valve. 



