298 THE VOYAGE OF H.M.S. CHALLENGER. 
American Jura-Trias (Jura only ?), which was first discovered by Meek and Hayden.’ It 
was subsequently found in Utah by Wheeler’s Survey, and owing to the constancy of its 
characters, even in examples collected at widely separate localities, it has been relied on 
with considerable confidence in the identification of Jurassic strata. According to Meek 
and Hayden there are five large, oval, petaloid areas, bounded on either side by “rather 
narrow, slightly elevated, transversely crenulated margins,”” and the figure which was 
published later by White’ clearly shows that the affinities of this species are with Hwtra- 
crinus rather than with Pentacrinus, of which it has been hitherto regarded as the only 
species yet recognised in the American Jurassic rocks. Hayden’s Survey found it in 
1877 extending through a considerable thickness of Jurassic limestones in Idaho and 
Wyoming.* The supposed Triassic beds of Dun.Glen and the Pah Ute range contain a 
slightly larger form of joint, which differs from: the Jurassic specimens. It was found 
associated with what are regarded as unmistakable Triassic fossils and also a Spirifera. 
Mr. Emmons’ speaks of it as follows:—‘ It should he stated also that these disks of Penta- 
crinus found in the Dun Glen limestone vary somewhat from the type specimens, and 
are all of larger size, reaching one-fourth of an inch in diameter, while those of Jurassic 
age scarcely reach one-fifth of an inch. Prof. Whitfield suggests that the Dun Glen 
variety may possibly be a new species.” 
Elsewhere, too, it is stated by Hall and Whitfield® that this Triassic form from Dun 
Glen differs from the Jurassic specimens ‘in the more obtuse points of the star, and 
the filling up of the angles between the points, and also in the broader form of the 
elliptical figures on the articulating surfaces of the disks.” These are just the differences 
between the stem-joints of Hxtracrinus and those of Pentacrinus, as explained above ; 
and I am therefore disposed to think that this Triassic type with broader petals may be 
a true Pentacrinus, more especially as we have no knowledge of any European Extra- 
crinus below the Lias. 
It must be remembered that nearly all of these identifications are based upon the 
characters of the stem-fragments only, the calyx.and arm-bases being but rarely preserved. 
Every one who has examined moderately long pieces of stem, whether recent or fossil, 
has noted the variation of characters which they present in different parts; and there 
has therefore been a general disposition not to place too much reliance on species of 
which only the stems are known. I have been surprised to find, however, in the case of 
1 Paleontology of the Upper Missouri, Invertebrates, part i. p. 67, pl. iii. fig. 2. 
* Proc. Acad. Nat. Sci. Philad., March 1858, p. 49. 
3 Report upon Geographical and Geological Explorations and Surveys west of the One Hundredth Meridian, in 
charge of First Lieut. Geo. M. Wheeler, vol. iv. part i., Palzeontology, p. 162, pl. xiii. fig. 6,a. 
4 Eleventh Annual Report of the U. S. Geological and Geographical Survey of the Territories, embracing Idaho 
and Wyoming, 1879, pp. 626, 627. 
5 Report of the Fortieth Parallel Survey, Descriptive Geology, vol. ii. p. 711. 
6 Thid., vol. iv. p. 280, pl. vi. fig. 16. 
