66 MESOZOIC AND CENOZOIC ECHINODERMATA OF THE UNITED STATES. 



Holectypus sp. Roemer, 1888, Pal. Abhandl. von Dames und Kayser, vol. 4, pt. 4, p. 9 (287), PI. I (XXXI), figs. 6a-c. 



Holectypus planatus Clark, 1891, Johns Hopkins Univ. Circ, vol. 10, No. 87, p. 76. 



Holectypus planatus Clark, 1S93, Johns Hopkins Univ. Circ, vol. 12, No. 103, p. 51. 



Holectypus planatus Clark, 1S93, U. S. Geol. Survey Bull. 97, pp. 58, 59, PI. XXII, figs. 2a-f. 



Holectypus charltoni Cragin, 1893, Geol. Survey Texas Fourth Ann. Rept., p. 158, PI. XXIV, figs 8, 9. 



Holectypus planatus Cragin, 1893, Geol. Survey Texas Fourth Ann. Rept., pp. 159, 160. 



Holectypus transpecosensis Cragin, 1893, Geol. Survey Texas Fourth Ann. Rept., p. 160, PL XXVII, figs. 3-5. 



Holectypus charltoni Cragin, 1894, Colorado Coll. Studies, vol. 5, p. 41. 



Holectypus planatus Hill, 1901, U. S. Geol. Survey Twenty-first Ann. Rept., pt. 7, p. 226. 



Determinative characters. — Test subcircular, subcorneal, flattened on the under surface. 

 Ambulacra narrow, straight, and somewhat lanceolate, with six irregular rows of tubercles; 

 poriferous zones somewhat straight, narrow, unigeminal. Interambulacra wide, plates numer- 

 ous and narrow, each with a nearly horizontal row of small tubercles. Apical system small, the 

 madreporite large. Peristome small, subcircular. Periproct large. 



Dimensions. — Transverse diameter 12-70 millimeters; height 7-40 millimeters. 

 Description. — This very common form from the Cretaceous of Texas was first reported by 

 Roemer in 1849, and was subsequently, in 1852, more accurately denned and figured by him. 

 Although wide differences in outline and in the size and arrangements of the tubercles occur, 

 they appear to be rather individual than specific. The majority of the specimens are subcir- 

 cular ; in the case of some, however, distinctly circular, while in that of others clearly pentagonal. 

 The ambitus in some individuals is sharp, in others rounded, whole the upper surface is dis- 

 tinctly elevated at the center with slightly tumid sides. The under surface is flat and slightly 

 depressed in the vicinity of the peristome. 



The ambulacra are narrow, straight, increasing in width toward the ambitus and somewhat 

 lanceolate in form. The poriferous zones are narrow, the pores small and unigeminal. Six 

 rows of small tubercles are found in the broader portion of the areas, which become reduced to 

 four toward the apical disk and peristome. 



The interambulacra are about three times the width of the ambulacra at the ambitus. The 

 plates are narrow, each with a horizontal row of small tubercles, five or six in a row at the am- 

 bitus, but fewer above and below. Each tubercle has a small circular areola, crenulated boss, 

 and perforated mamelon. The tubercles cover the greater portion of the plates, with minute 

 granules interspersed between them. The tubercles are very much larger on the lower surface, 

 and the difference in size shown between those of the ambulacra and interambulacra on the 

 upper surface disappears. 



The apical system is small. The five ovarial plates are all perforated; the madreporite is 

 of large size and indistinctly separated from the other plates. 



The peristome is small, with slight incisions that give a decagonal margin. The large oval 

 peristome extends from the vicinity of the mouth to the ambitus. 



Related forms. — The size of the adult forms is very variable, those found in the Washita 

 group being larger than those found in the Trinity and Fredericksburg group. The individuals 

 also vary in height. So many connecting forms can be found between the typical H. planatus 

 and the species described as H. charltoni and H. transpecosensis by Cragin that they have been 

 regarded as synonyms. 



Localities. — Near Fredericksburg (type), Cow Creek, Travis County (figured specimens A 

 and B) ; Denton County (figured specimen C) ; Shoal Creek, Austin (figured specimen D) ; San 

 Antonio, Kent, and many other localities in Texas. 



Geologic horizon. — Trinity, Fredericksburg, and Washita groups of the Comanche series, 

 Cretaceous. 



Collections. — U. S. National Museum (12236); Academy of Natural Sciences of Phila- 

 delphia; Boston Society of Natural History ; Geological Survey of Texas; Johns Hopkins 

 University (T 300S). 



