GIRTY, INVERTEBRATE FOSSILS FROM COLORADO 5 



Of described species, this appears most closely to resemble A. gilberti, 

 though it is not certain that the two are congeneric. The chief difference 

 of a possible generic character lies in the fact that White's figure ap- 

 pears to represent A. gilberti as having a well-marked escutcheon, a 

 structure probably not present in A. squamulifera. Specifically, the 

 latter appears to be a more slender form, more convex, and with a sharper 

 umbonal ridge (these characters, however, may be enhanced by compres- 

 sion in the Colorado form). It is also distinctly, though finely, costate, 

 although A. gilberti in fact is covered with granules arranged in rows, so 

 as to resemble minute radiating lirse. 



Horizon and locality: Lykins formation; Hey good Canyon (lots 

 3264, 3265, 3266) and Perry Park (lot 3263), Colorado. 



Alula gilberti White? 



Alula squamulifera is abundant in lot 3263, but specimens are in an 

 unsatisfactory condition of preservation. Many of them show a lower 

 convexity and less angular umbonal ridge than the types. One example 

 is sufficiently shallow, Broad and ill-defined as to umbonal ridge to re- 

 semble Allerisma gilberti rather closely. The sculpture is obscure but 

 presents suggestions of radiating costae or of rows of papillae. The de- 

 pressed specimens which are provisionally placed with A. squamulifera 

 appear to show a gradation toward but not into the only one referred to 

 White's species, and the facts which I have been able to observe leave me 

 in doubt as to whether we have three species of not necessarily generically 

 identical shells, or a fairly continuous series of mutations with A. squa- 

 mulifera at one end and A. gilberti (or the form here identified as such) 

 at the other. 



Horizon and locality: Lykins formation; Perry Park, Colorado 

 (lot 3263). 



Myalina wyomingensis Lea 



Myalinas are extremely abundant in four of the five collections exam- 

 ined, but most of the specimens are small. They vary in specific charac- 

 ter. Some of the larger and more typical specimens agree in every 

 determinable character with forms from the Eico formation of the San 

 Juan region which I identified as Myalina wyomingensis. 41 The great 

 majority are of much smaller size, more like the form from Ouray which 

 I somewhat provisionally called M. cuneiformis. 5 They naturally have 

 the anterior lobe less strongly developed than the larger or mature exam- 

 ples which accompany them. They seem as a rule to be less strongly 



* U. S. Geol. Survey, Prof. Paper 16, Plate VIII, Fig. 8. 1903. 

 B Ibid., Plate VIII, Figs. 16 and 17. 



