NO, 4 CRETACEOUS BRACHIOPODA, ARIZONA — COOPER II 



This genus, although not named until now, was recognized by 

 Deslongchamps (1884, p. 189) in his discussion of the genus Zeilleria. 

 He characterizes the division as having a relatively short, thick beak 

 having a very large foramen. The shell is globular, short, and compact. 

 It is unique in the Cretaceous. 



PSILOTHYRIS OCCIDENTALIS Cooper, new species 

 Plate 3B, figures 4-24; plate 4A, figures i, 2 



Shell small, attaining a length of five-eighths inch; outline sub- 

 pentagonal with the length slightly greater than the width; greatest 

 width located slightly posterior to the middle ; sides sloping medially ; 

 anterior margin subtruncate ; posterior margin forming an obtuse 

 angle. Anterior commissure uniplicate ; lateral commissure straight. 

 Valves unequal in depth, the pedicle valve deeper; surface smooth 

 except for concentric lines and varices of growth. 



Pedicle valve strongly convex in lateral profile, with the maximum 

 convexity slightly posterior to the middle ; anterior profile strongly 

 convex ; umbonal region inflated ; beak small, erect ; beak ridges strong. 

 Median region swollen; anterior slope flattened; flanks swollen and 

 steep. Foramen small, round, mesothyrid, slightly labiate. Deltidial 

 plates conjunct, suture visible. Interior of pedicle valve with large 

 teeth supported by stout dental plates. No pedicle collar. Muscle 

 marks lightly impressed. 



Brachial valve in lateral profile flattened in the median region but 

 convex at the posterior and anterior; anterior profile broadly and 

 gently convex. Umbonal region swollen but median area flattened; 

 flanks narrowly convex. 



Interior of the brachial valve with short, undivided hinge plate 

 deeply excavated anteriorly and thickened, elevated or puckered on the 

 anterior edge. Socket ridge short, stout. Crura short ; crural processes 

 long and slender in the adult loop but short and blunt in the young. 

 Loop long and free in the adult ; loop short, broad, and attached to a 

 short septum on the floor of the valve in the young. Septum in the 

 adult short and confined to a position at the beak and under the hinge 

 plate and may or may not support the hinge plate. 



Development of the loop. — In the smallest specimen available, 

 measuring 3 mm, in length and probably the same in width (paratype 

 U.S,N,M, No. 124190k), the notothyrial region is deeply concave 

 and without a hinge plate. The crura are slender and arise from ridges 

 bordering the notothyrial cavity. The descending lamellae are short 

 and their anterior ends converge to unite with a septal blade or pillar 



