JURASSIC PERIOD. 168 



there are smaller joints, varying from 0.05 to 0.10 inch in diameter, having 

 proportionally shorter and broader rays, which are usually less angular at 

 the points than the broader ones are." 



All the specimens of this species contained in the collections consist of 

 portions of the column; and, up to the present time, nothing more has been 

 learned concerning the characteristics of the species. Generally, these por- 

 tions of the Crinoidea are of very little value in specific disciiminations ; 

 but the characters of those parts of this species are so constant, even in 

 examples collected at -widely-separated localities, that it has been relied 

 upon with considerable confidence in the identification of Jm-assic strata. 

 This is the only species of the genus Fentacrinus yet recognized in the 

 Jurassic rocks of America, but it is not improbable that others may here- 

 after be discovered in them. The largest examples in the collections have 

 a diameter one-third greater than that of the largest of those mentioned by 

 Meek and Hayden, and they also present some slight variations from the 

 latter. The principal difference is the alternation, at irregular intervals, of 

 joints that are almost pentahedral with those that are deeply pentalobate. 

 This character is shown in one of the figures on Plate XIII, but it is proba- 

 bly not a specific one. 



Position and locality. — Strata of the Jurassic period : Salt Creek, near 

 Nephi ; and Diamond Valley, near Saint George, Utah. It has quite an 

 extended geographical range, having been also discovered in Idaho. 



MOLLUSC A. 

 Class CONCHIFERA. 



Order MONOMYARIA. 



Family OSTREIDiE. 



Genus OSTREA Linnfeus, 1758. 



Ostrea strigilecula White. 



Plate XIII, fig. 3 a, h, c, and d. 



Shell small, irregularly suboval in outline ; axis much curved, making 

 the ventral border broadly arcuate and the dorsal border more or less 



