DESCRIPTIONS OF SPECIES. 277 



1899. Orfhotheles InxquaVis. Girty, U. S. Geol. Surv., Mon., vol. 32, pt. 2, p. 522, pi. 68, fig. 3a. 

 Madison limestone: Yellowstone National Park. 



1900. Ortlwthetes inicqaalis. AVeller, Acad. Sci. St. Louis, Trans., vol. 10, p. 66, pi. 1, fig. 18. 

 Chonopectus sandstone: Burlington, Iowa. 



1901. Orthothetes imequalis. Weller, Acad. Sci. St. Louis, Trans., vol. 11, p. 159, pi. 14, figs. 16-18. 



Kinderhook: Bed 4, Burlington, Iowa. 

 1901. Orthothetes inivqualls/. Weller, Acad. Sci. St. Louis, Trans., vol. 11, p. 195, pi. 19, fig. 9. 

 Kinderhook: Bed 7, Burlington, Iowa. 



The material representing Orthothetex in the San Juan region, recently added to 

 our collections, far exceed.s in quantity- all that had been previou.'3ly obtained from 

 other portions of the State, and of course, though often fragmentary, offers a much 

 more complete presentation of the specific chai'acters. I believe that I am safe in 

 referring all of this material to the same species. Nevertheless, because of its scanty 

 representation in all the areas but one, each set of sjjecimens will be considered 

 separately. 



In the San Juan region, as just intimated, the form seems to be rather plentiful 

 at a number of localities. I feel little hesitation in expressing the opinion that it 

 is the same species which, in .studying the fauna of the Madison limestone (of 

 Yellowstone Park) I referred to OrthotheUs uuvijiatlis. The form is of medium size, 

 transversely semicircular, with subquadrate cardinal angles. The dorsal valve is 

 moderately inflated, the ventral slightly concave or nearlj' plane. The ventral area, 

 which is not very high, makes a more or less acute angle with the general surface of 

 the valve. The ventral beak is small and ill defined, but produces in the cardinal 

 outline a broadly obtuse-angular shape. The surface is crossed hj numerous, very 

 slender radiating stria?, the spaces between which, considerably wider than the 

 elevations themselves, are traversed by delicate concentric stria?. The radiating 

 striie vary considerably in coarseness, but usually show a pretty regular alternating 

 inequalit}-, especially on the lower areas of the shell. The immber of striaj varies in 

 different individuals from y to 14 in 5 mm., depending somewhat upon the part of the 

 shell in which the measurement is taken. The same I'ange of variation occurs in Yellow- 

 stone Park, and as it can frequently be found at one locality with the intermediate 

 conditions abundantly represented, this character does not, in my judgment, furnish a 

 satisfactory basis for subdivision into species or even varieties in this case. In the 

 matter of size a transverse diameter of 40 mm., though not infrequent, is above the 

 average. 



From the Crested Butte region there is only a single specimen, an imperfect 

 external cast of a ventral valve. It differs from characteristic specimens of Derhya 

 crmsa in being much lai'ger and having the striation finer and .somewhat less alter- 

 nating. It agrees in all the characters preserved with Orthothetes insequalis as seen 

 in specimens from the Madison limestone. 



