110 BULLETIN 103, UNITED STATES NATIONAL MUSEUM. 



Encope platytata is a near ally of Encope tenuis Kew x of the 

 Miocene of California, but differs from that species in that the great- 

 est height of the test is central, and the periproct is confluent with 

 the lunule. 



Locality and geologic occurrence. — Gatun formation, Miocene, 

 Panama Canal Zone, from lowest horizon in big cut, one-fourth to 

 one-half mile beyond Camp Cotton toward Monte Lirio, D. F. Mac- 

 Donald and T. W. Vaughan, collectors, 1911, U. S. National Museum 

 station No. 6029#, one specimen. 



Type.— Cat. No. 324455, U.S.N.M. 



ENCOPE MEGATREMA Jackson. 



Plate 52, fig. 1. 

 Encope megatrema Jackson, Proc. U. S. Nat. Mus., vol. 53, 1917, p. 496, 

 text figs. 3, 4 ; pi. 68, fig. 1. 



This species is represented by one fairly good test with its counter- 

 part, v and in addition some 12 fragments which yield helpful facts 

 on close study. From the incompleteness, measurements and some 

 details will have to be given in general terms or omitted. As a whole, 

 the test is low, elongated, thin on the borders and with shallow 

 ambulacra! notches and an enormous lunule in interambulacrum 5. 



From the best specimen, which is figured, the length probably was 

 about 120 mm. and the width about 106 mm.; thickness of the test 

 at its center is 10 mm. Ambulacral notches are shallow and quite 

 wide in areas II and V, indicating that this is the character in the 

 two posterior ambulacra I and V and also in the paired anterior am- 

 bulacra II and IV. This evidence is supported by several of the frag- 

 ments which show shallow lobes like the type, but it can not be 

 definitely stated which areas they represent. The notch of the ante- 

 rior odd ambulacrum III is not known, but it was probably shallower 

 than the others, as is characteristic of species of the genus. The most 

 striking feature of this species is the lunule in interambulacrum 5, 

 which is enormous. It is situated about midway between the apical 

 disk and posterior limits of the test, and is roughly triangular in 

 shape, the apex of the triangle pointing anteriorly. It measures at 

 the surface of the opening 27 mm. in length and 27 mm. in width at 

 the widest part posteriorly. The walls of the lunule slope outward 

 from the center, as seen looking from above, as is well shown in two 

 of the fragmentary specimens. From this sloping character of the 

 walls, it results that the width of the lunule would be greater by about 

 6 to 10 millimeters on the ventral side than it is on the dorsal. The 

 height of the wall of the lunule is 12 mm., which is doubtless the 

 highest 'point of the test. The lunule in this species is, relatively to 

 the size of the specimens, the largest known in any species of the 



1 Kew, W. S. W. Tertiary echinoids of the Carrizo Creek Region in the Colorado Desert. 

 University of California Bull., Dept. Geology, vol. S, No. 5, pp. 39-60, pis. 1-3, 1914. 



