NO. 4 CRETACEOUS BRACHIOPODA, ARIZONA — COOPER I3 



about their middle. The septum has been absorbed to a mere remnant 

 which extends for a short distance only anterior to the hinge plate. 

 Except for the incompletely developed hinge plate and the remnants 

 of the septal attachments, the loop is essentially adult in character. 

 This stage is the dalliniform stage. 



By summarizing the evidence from these f ev^ specimens it is possible 

 to give a fairly complete account of the loop development. Prior to 

 3 mm. the median septum must show as a small projection from the 

 floor. At 3 mm. the septum has become elongated and the descending 

 branches have grown anteriorly to meet the sides of the elongated free 

 distal expanded part of the septum and the ring bud starts to develop. 

 By 42 mm. the expanded end of the septum splits laterally and the 

 ring enlarges, the septum having a deep reentrant anteriorly and the 

 descending branches extended a considerable distance anteriorly. At 

 6.6 mm. the loop is strong, with broad descending branches extended 

 beyond the anterior end of the septum, and the loop attachment to the 

 septum is a small process. At 7.6 mm. the loop is now free of the 

 septum which has become nearly completely resorbed except for the 

 short remnant supporting the hinge plate. Remnants of the process 

 attaching the loop to the septum can be seen in specimens having at- 

 tained a length of 12.5 millimeters. 



The hinge plate is deeply concave in the young, but in the '].(i mm. 

 stage the transverse plate forming the flat and undivided hinge plate 

 forms, and this lengthens with advancing age. In old age it becomes 

 puckered or upturned on its free edge. 



MEASUREMENTS IN MILLIMETERS 



Length 



Holotype U.S.N. M. No. 124191 15. i 



Paratype U.S.N.M. No. 124189 13.3 



" " " 124190a 11.4 



" " " 124190b 12.4 



Types. — Holotype U.S.N.M. No. 124191 ; figured paratypes 

 U.S.N.M. Nos. 124190a, c, e, f, h-j, 124202a, d-i ; measured paratypes 

 U.S.N.M. Nos. 124189, 124190a, b; described but unfigured paratypes 

 U.S.N.M. No. 124190k; unfigured paratypes U.S.N.M. Nos. I24i90d, 

 g, 124202b. 



Horizon and locality. — Mural limestone, from a small hill 300 yards 

 east of U. S. Highway 80, NWiSW^NEi sec. 36, T. 23 S., 



