298 THE VOYAGE OF H.M.S. CHALLENGER. 



American Jura-Trias (Jura only 1), 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 Extra- 

 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 be 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 HaU 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 difi"erences 

 between the stem-joints of Extracrinus 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. 

 Ever}'^ one who has examined moderately long pieces of stem, wdiether 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 jilace too much reliance on sj^ecies of 

 which only the stems are known. I have been surj^rised to find, however, in the case of 



^ Palaaontology of the Upper Missouri, Invertebrates, pari i. p. 67, pi. iii. fig. 2. 



2 Proc. Acai. Nat. Sci. Philad., March 1858, p. 49. 



2 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., Pahvontology, p. 162, pi. xiii. fig. 6,a. 



' Eleventh Annual Report of the U. S. Geological and Geographical Survey of tlie Territories, embracing Idaho 

 and Wyoming, 1879, pp. 626, 627. 



^ Report of the Fortieth Parallel Survey, Descriptive Geology, vol. ii. p. 711. 



6 Hid., vol. iv. p. 280, pi. vi. fig. 16. 



