CRETACEOUS ECHINODERMATA. 55 



The peristome is of medium size with moderately developed incisions. The periproct is 

 relatively large, the plates of the apical system being narrow. 

 Locality. — Sierra Blanca Mountain, near El Paso, Tex. 

 Geologic horizon. — Washita group of the Comanche series, Cretaceous. 

 Collection. — Geological Survey of Texas. 



Family DIADEMATID.E. 



Genus PSETJTDODIADEMA Desor. 



PSETJDODIADEMA TEXANUM (Roemcr). 



Plate XVIII, figures la-i. 



Diadema texanum Roemer, 1852, Die Kreidebildungen von Texas, p. 82, PL X, fig. 5. 



Pseudodiadema texanum Desor, 1S58, Synopsis des echinides fossiles, p. 72. 



Diadema texanum Gabb, 1859, Catalogue of invertebrate fossils of the Cretaceous formation, p. 19. 



Pseudodiadema texanum Meek, 1S64, Smithsonian Misc. Coll., vol. 7 (177), p. 2. 



Pseudodiadema roemeri Clark, 1S91, Johns Hopkins Univ. Circ, vol. 10, No. 87, p. 75. 



Pseudodiadema texanum Clark, '1S93, Johns Hopkins Univ. Circ, vol. 12, No. 103, p. 51. 



Pseudodiadema texanum Clark, 1893, U. S. Geol. Survey Bull. 97, pp. 47, 48, Pis. XIII, figs. 2a-n; XIV, figs. la-g. 



Pseudodiadema texanum Cragin, 1893, Geol. Survey Texas Fourth Ann. Rept., pp. 161, 162. 



Pseudodiadema texanum Hill and Yaughan, 1898, U. S. Geol. Survey Eighteenth Ann. Kept., pt. 2, PL LIII, figs. 3a, 3b. 



Pseudodiadema texanum Hill, 1901, U. S. Geol. Survey Twenty-first Ann. Rept., pt. 7, PL XXVII, figs. 3a, 3b. 



Determinative characters. — Test small, circular, depressed; sides inflated; upper and lower 

 surfaces about equally flattened. Ambulacra narrow, with two rows of primary tubercles, 11 

 or 12 in each row; poriferous zones narrow, pores uniserial. Interambulacra wide, with 

 two rows of primary tubercles of larger size than those of the ambulacra. Peristome wide, cov- 

 ering nearly one-half the diameter of the test. Periproct subcircular, with deep incision in right- 

 anterior ambulacrum. 



Dimensions. — Diameter 5 to 12 millimeters; height 8 to 22 millimeters, 



Description. — This form, described by Roemer in 1852 as Diadema texanum, was properly 

 assigned by Desor to the genus Pseudodiadema. 



The test is circular, very much depressed on both the upper and lower surfaces and inflated 

 at the sides. 



The ambulacra are narrow and lanceolate. They bear two rows of tubercles, 11 or 

 12 in each row, that decrease rapidly hi size from the ambitus to the poles. The areolas are 

 circular, the bosses stout and prominent, the mamelons distinctly perforated. Small secondary 

 tubercles are irregularly dispersed between the rows of primary tubercles. A few scattered 

 granules surround the same. The poriferous zones are uniserial throughout, though the pores 

 show a slight tendency to increase at the peristome. 



The interambulacra are about twice the width of the ambulacra. There are two rows of 

 primary tubercles, 9 or 10 in each row, that decrease in size from the ambitus to the poles. 

 The areolas are depressed, circular, and distinctly outlined, the bosses elevated and deeply crenu- 

 lated, the mamelons large and deeply perforated. Small secondary tubercles surround the pri- 

 mary tubercles, between which are interspersed minute granules. 



The peristome is wide, covering nearly one-half of the diameter of the test. Ten incisions 

 produce well-defined lobes at regular intervals. The periproct is large and subcircular, with a 

 deep incision in the right anterior ambulacium. 



Related forms. — Both Roemer and Desor mention the similarity of this species to P. tenue, 

 of the Cenomanian of Europe, from which, however, it is separated by its smaller tubercles and 

 more numerous granules. 



Localities. — Near Fredericksburg and in San Saba Valley (type), Barton Creek between Oat- 

 man ville and Bee Caves (figured specimen), near Austin and many other places in Texas. 



Geologic horizon. — -Glen Rose limestone of Trinity group and Comanche Peak limestone of 

 Fredericksburg group, Comanche series, Cretaceous. 



Collections. — -U. S. National Museum (31197); Johns Hopkins University. 



