DESCRIPTIONS OF SPECIES. 381 



M. liaydenensis in some of its phases resembles M. VMbashensis, especially the 

 geniculate vai-ietj*. One, the least characteristic specimen seen, can not be distin- 

 g-uished, except for size, from geniculate shells belonging to M. u^aiashensis, but 

 average, and cei'tainly well characterized, examples are smaller, less transvei'se, 

 much more inflated, and with stronger submarginal ridges. The Colorado specimens 

 referred to Jf. loalMsliensis are all, as I have taken occasion to remark, of the genic- 

 ulate tj^pe, and somewhat smaller than is common in the Mississippi Valley. They 

 are extremely similar to M. Jmydenensis when the latter is in its least typical expres- 

 sion. Ijut they do not assume the characters of typical J/, haydenensis, and probably 

 as a rule — certainlj' in many cases — begin the development of submarginal ridges at a 

 much earlier period of their growth. Since the shell preserved in the average speci- 

 men is terminated by the submarginal ridges, the form referred to 3£. tuahashensis 

 would be less highly arched and would very much less transgress the hinge line than 

 typical M. haydenensis. 



Locality. — Grand River region (station 23iirf:), Grand River (station 2325). 



SPIRIFER Sowerby, 1815. 



Spirifek boonensis Swallow ?. 



PI. VI, figs. 1-lb, 2, 2a, 3. 



1860. Spirifer boonensis. Swallow, Acad. Sci. St. Louis, Trans., vol. 1, p. 646. 

 Lower Coal Measures: Boone, Randolph, and Monroe counties. Mo. 



The Upper Cai'boniferous spirifers in our collection from Colorado can be 

 assembled into three specitic groups. One of these belongs to the ,type usually 

 referred to Sp. cameratus Morton. Another is the form for which Marcou proposed 

 the name Spirifeu' rockymontanm., and wliich Hall almost simultaneously described 

 as 8p. op'nnm. The third I have referred rather doubtfully to Sp. loo^iensis Swal- 

 low. It is distinguished from Sj). rockymontaivus by being larger and by having an 

 extended hinge line. The largest specimen of Sp. rockymontanua observed by me 

 has a transverse diameter of 26 mm., and the average individual has dimensions 

 considerably less. A large specimen of Sp). hocmensis measures 40 mm., though this, 

 too, is ."omewhat above the average. 



Sj^. hoonen^i'i, or the form which I have referred to that species, is of the hnhrex 

 t^'pe, transver.se, with more or less extended cardinal extremities. Growth in older 

 •specimens is often by anterior and anterolateral additions, which gives the shell a 

 subquadrate shape, but the hinge line remains characteristically its longest diameter. 

 There are 4 to 6 plications on the fold and 3 to 5 on the sinus, with 12 or 13 lateral 

 ones. These are ci'ossed by a system of very fine imbricating concentric and I'adiat- 

 ing striae, the latter being frequently the more prominent. 



Sp. Imonen-sis is known only from the original description, and it has not been 



