CEETACEOUS ECHINODEEMATA. 61 



subpentagonal ambital outline. If the differences are more than varietal, then C. texanum must 

 at all events be regarded as the ancestor of C. volanum. 



Localities. — Fredericksburg and in the San Saba Kiver valley (type), Leon Springs (figured 

 specimen), near Austin, and many other places in Texas. 



Geologic horizon. — Comanche Peak limestone cf Fredericksburg group, Comanche series, 

 Cretaceous. 



Collections. — U. S. National Museum (9838); Boston Society of Natural History; Geologi- 

 cal Survey of Texas ; University of Iowa; Johns Hopkins University. 



Cyphosoma volantjm Cragin. 

 Plate XXI, figures 2a-b, 3. 



Cyphosoma volanum Cragin, 1893, Geol. Survey Texas Fourth Ann. Rept., p. 147. 



Cyphosoma volanum Cragin, 1894, Colorado Coll. Studies, vol. 4, pp. 45, 47. 



Cyphosoma volanum~H\H, 1901, U. S. Geol. Survey Twenty-first Ann. Rept., pt. 7, p. 283. 



Determinative characters. — Test small, subcircular; upper surface slightly elevated; lower 

 surface depressed, concave. Ambulacra slightly prominent, with two rows of primary tubercles ; 

 poriferous zones sinuous; pores uniserial from peristome nearly to apical system. Inter- 

 ambulacra wide, each with two rows of primary tubercles, similar to those of ambulacra, adjoin- 

 ing which are smaller primary or secondary tubercles. Peristome small. Periproct large. 



Dimensions. — Diameter 17.5 millimeters; height 8 millimeters. 



Description. — This small species has a subcircular test with a slightly elevated upper surface. 

 In general size and appearance the smaller specimens resemble Pseudodiadema texanum, but 

 their other characters are very different. 



The ambulacra are broad, with two rows of primary tubercles eight or nine in each row. 

 The tubercles are large at the ambitus but become very small toward the apical system and peri- 

 stome. The poriferous zones are narrow on the lower surface and at the ambitus, but widen above. 

 They are sinuous. The pores are uniserial, although crowded toward the apical system. 



The mterambulacra are wide. There are two rows of primary tubercles similar to those of 

 the ambulacra extending from the peristome to the apical system. Additional smaller primary 

 or secondary tubercles, somewhat irregularly dispersed, adjoin the primary tubercles, while 

 numerous granules occur on the surface of the plates. 



The peristome is small with distinct branchial incisions. The periproct is large and sub- 

 pentagonal. 



Related forms. — Few if any differences serve to separate this form from C. texanum of the 

 Fredericksburg group. In general it is lower and more depressed, is rather more subpentagonal 

 in outline, and the four rows of tubercles on its interambulacra are less fully denned in many 

 specimens. If more than a variety, it is certainly very closely related to C. texanum and de- 

 scended from it. 



Localities. — Below the Missouri, Kansas & Texas Railway bridge on Big Elm Creek, Benton 

 County (type), and Denison, Grayson County, Tex. 



Geologic horizon. — Denison formation of Washita group, Comanche series, Cretaceous. 



Collection. — Geological Survey of Texas (A, B). 



Genus COPTOSOMA Desor. 



Coptosoma speciosum Clark. 



Plate XIX, figures 3a-h. 



Cyphosoma speciosum Clark, 1891, Johns Hopkins Univ. Circ. , vol. 10. No. 87, p. 70. 

 Coptosoma speciosum Clark, 1893, Johns Hopkins Univ. Circ, vol. 12, p. 51. 

 Coptosoma speciosum Clark, 1893, U. S. Geol. Survey Bull. 97, pp. 52, 53, PL XVIII, figs. la-h. 

 Coptosoma speciosum Johnson, 1905, Acad. Nat. Sci. Philadelphia Proc, vol. 57, p. 6. 



Pseudodiadema speciosum Weller, 1907, Cretaceous paleontology of New Jersey: Geol. Survey New Jersey, pp. 280- 

 288, PL IX, figs. 7-14. 



Determinative characters. — Test circular, inflated at the sides, flattened on the upper and 

 lower surfaces. Ambulacra wide, with two rows of primary tubercles, twelve or thirteen in 

 39800°— 15 5 



