PLATE U— Continued. 



OBOLELLA, Billings. 



Page 66. 



Obolella crassa, Hall. 



Fig. 31. The extei'ioi- of ;i valve. Natural size. 



Fig-. '62. T-he same, enlarged ; ."showing the radiating striiE between the lauiellose growth-lines. X 2. 



•Fig. 33. Anothei- valve, similarly enlarged, in which the radiating lines do not appear. This is the speci- 

 men originally described as Avicida dtiiquaiimta. Hall (Pal. N. Y., vol. i. 1847). 



Fig. 34. The interioi- of a pedicle-valve ; showing the pedicle-groove, the caiilinal. lateral and centi-al mus- 

 cular scars. X 2. 



Fig. 35. The interior of another, but .smaller, pedicle-valve ; showing the same features. X 2. 



Fig. 36. The interior of the brachial valve ; showing the faint pedicle-groove, the cardinal, extei-nal, cen- 

 tral and lateral muscular impressions. X 2. 



Middle Cambrian. Near Troy, N. ¥. 



ObOLELLA rOLITA, Hill I. 



The exterior of a valve. X 4. .^ ^^ vi i.  



The interior of a pedicle-valve ; showing the cent)-al and lateral muscular impressions. X 2. 

 An internal cast of the opposite (<) valve. X 2. 

 The interior of a brachial \alve X 6. 

 The interior of a pedicle-valve. X 6. 

 (Figs. 40 and 41 are from the original di-awings representeil in the Sixteenth Rept. N. Y. State 

 Cab. Nat. Hist., pi. vi. tigs. 20, 21.) 



Potsdam sandstone. Trempaleau, Wisconsin. 



Obolella gemma, Billings. 



Fig. 42. The exterior of a pedicle-valve. X ti. 



Fig. 43. (34 in eri'oi-, last line of figures.) The interior of a brachial valve ; showing the musculai- im- 

 pressions. X 6. 



Fig. 44. The intei-ior of a pedicle-vahe ; showing the cardinal area, pedicle-groove and muscular im- 

 pressions. X 6. 

 (The originals of fig.s'. 1, 2, 4, 34, 3.5, 36, 42, 43, 44, are in the collection of Mr. S. W. Ford.) 



