228 University of Texas Bulletin 



form with their superior border approximately the prolongation of the 

 cardinal border and do not elevate themselves above the beaks. The beaks 

 are small and pointed. The cardinal border is straight, long, and vertical 

 in relation to the longitudinal axis of the shell. The surface of the shell 

 is entirely smooth and shows only fine concentric striae of growth. All 

 our specimens are internal molds but some have preserved remains of the 

 shell. 



Our very characteristic species resembles Av icula gravida Coquand. 1 The 

 main difference seems to consist in the greater convexity of the valves in 

 this latter species, also in the different position of the beaks in relation to 

 the upper border of the auricula and the cardinal border ; but it is possible 

 that this last difference may be explained as an error of the draftsman, 

 because Coquand mentions expressly that neither the auricula nor the pos- 

 terior wing elevate themselves above the beaks. In every case our species 

 is nearly related to the African one. 



Coquand mentions that his species occurs in the Turonian (Mornasian) 1 ; 

 later on Peron 2 corrected this determination of the horizon. He says that 

 he has always found this species in the Santonian, at the ^ase of the Se- 

 nonian, in the beds with Buchiceras and Hemiaster Fourneli. But not- 

 withstanding this correction it is possible that Coquand was right in his 

 determination of the horizon, because Pervinquiere 3 also mentions that he 

 has found Avicula gravida in the lower Turonian. The species seems to 

 occur also in the Emscherian, from which horizon it is cited by Pervin- 

 quiere 4 who has also found it at the base of the Senonian immediately 

 above the Turonian. The species thus probably lived from the Salmurian 

 to the Senonian; it is of course possible that the specimens found in the 

 different horizons really were different species, although these certainly 

 belong to the same group. 



At first view, one might believe that our species are identical with Avi- 

 cula pedernalis Roemer 1 but Avicula Aguilera does not have the singular 

 scars of that species, which remind us of those of Meleagrina. Roemer 

 has figured them and I have observed them also on a specimen from the 

 Vraconian of Arivechi, Sonora. The general form of Roemer's species is 

 also a little different from ours. 



Number of specimens : 6. 



Age: Lower Turonian (Salmurian), upper horizon. 



Coquand, Geol. et Pal. de Constantine, p. 216, pi. 13, figs. 17, 18. 

 2 Thomas et Peron, Hauts-Plateaux de la Tunisie, p. 241. 

 3 Pervinquiere, Et. geol. Tunisie, pp. 101, 108. 

 4 Pervinquiere, Et. geol. Tunisie, p. 115, 117, 151. 



