34 MESOZOIC AND CENOZOIC ECHINODERMATA OF THE UNITED STATES. 



unknown. The ambulacra are very distinct, the poriferous zones showing clearly.. The ambu- 

 lacra are narrow, increasing in width gradually to the ambitus. The interambulacra stand out 

 prominently, the sutures between the plates being clearly indicated. 



Locality. — North of Berry Creek, Yellowstone National Park, Wyo. 



Geologic horizon. — Jurassic. 



Collection.— V. S. National Museum (31192). 



Holectypus cragini Clark, n. sp. 

 Plate V, figure 4. 

 Holectypus ? sp. Cragin, 1905, U. S. Geol. Survey Bull. 266, p. 35. 



Determinative characters. — Test subcircular, subconical. Ambulacra narrow, straight, with 

 several rows of tubercles; poriferous zones straight and narrow. Interambulacra wide. Apical 

 system imperfect. 



Dimensions. — Diameter 28 millimeters; height 15 millimeters. 



Description. — The fragmentary specimen referred to by Cragin is an unquestioned Holec- 

 typus and presents many of the diagnostic characters of that genus. It is not unlike II. plahatus 

 in general outline, but its much earlier geologic position renders it necessary to place it in an inde- 

 pendent species. The test is much damaged, so that its essential specific characters can not be 

 determined. The outlines of the ambulacra and interambulacra can be seen, but the individual 

 plates are with difficulty distinguished. The ambitus is relatively sharp, indicating a flat under- 

 surface. The tubercles in this species are evidently small and cover the greater portion of the 

 plates. The apical system is small but the individual plates can not be distinguished. 



Related forms. — This species is wholly unlike H. peali in form and is evidently a different 

 species. 



Locality. — Malone Mountain, Texas. 



Geologic horizon. — -Malone formation, Upper Jurassic. 



Collection.— U. S. National Museum (31193). 



Order ATELOSTOMATA. 



Suborder ASTERNATA. 



Family CASSIDULID^I. 



Genus PYGXTRtTS Agassiz. 



Pygurus sp. 

 Pygurus sp. Cragin, 1905, U. S. Geol. Survey Bull. 266, p. 36. 



Description. — Cragin describes the form as follows: 



Among the specimens submitted by Dr. Stanton is a fragment of the test of a sea urchin of the genus Pygurus. It 

 includes the main part of the aboral half of an ambulacrum. The genus is clearly determined by the form of the 

 ambulacrum and the form and arrangement of the pores, as well as by the character of the tubercles and by the dis- 

 coidal form of the test, the latter form being indicated by the flatness of the part preserved. The ambulacrum has the 

 biconcave outline that characterizes the upper part of that of Pygurus. The pores of the outer row are very long and 

 slitlike, widened slightly toward the outer end, and subhorizontal to more or less oblique; those of the inner row more 

 ringent, compressed dotlike or hyphen-like, and oblique, their width being considerably less and their length consid- 

 erably more than that of any of the circumtubercular courts on the neighboring part of the ambulacrum. The ambu- 

 lacral plates are exceedingly narrow; the imperforate part of each, in the widest part of the ambulacrum, being about 

 10 times as long (transverse) as wide (parallel to the course of the ambulacrum) and ornamented with 1 to 3 small 

 perforated tubercles, each of which is set in a round, depressed court. The distinction of these tubercles is irregular, 

 but they are so few and so feebly developed on the inner ends of the plates as to give the ambulacrum the aspect of 

 having a median plain zone, to which the seams between the plates give a transversely striated appearance. 



Measurements.— Maximum width of ambulacrum 16, of which the two pore belts each occupy 3.5 and the imper- 

 forate tract 9 millimeters; length and width of each imperforate part of an ambulacral plate in broadest part of the 

 ambulacrum, respectively, 4.5 and 0.43 millimeters; length of outer slitlike pores in broadest part of ambulacrum 2 

 millimeters, which is about one-fourth of the width of a semiambulacrum. 



Occurrence.— West side of Malone Mountain, about 2 miles southwest of Malone station, in No. 25 of Dr. Stanton's 

 Malone Mountain section; with Gryphsea mexicana, Pleuromya inconstans, Pinna quadrifrons, etc. 

 Geologic horizon. — Jurassic. 

 Collection. — U. S. National Museum. 



