0.20 mm; biconvexity 0.18 mm. 



Occurrence: Black Creek only, localities G, H, 

 I, J, and K. U.S.N.M. 129029, 129030, 129031. 



Remarks: This species most closely resembles 

 0. scorbiculata Alexander but the arrangement of 

 the tubercles serves to distinguish this species. 

 These specimens like O. cristata Alexander exhibit 

 a radial reticulate pattern on and around the 

 base of the tubercle. 



Orthonotacythere hannai (Israelsky) 

 Plate 6, Figures 3-5 



Cytheridea (?) hannai Israelsky, 1929, Arkan- 

 sas Geol. Bull. 2, p. 12, pi. 2A, fig. 10. 



Cytheropteron hannai (Israelsky) Alexander, 

 1929, Texas Univ. Bull. 2907, p. 105, PI. 9, 

 fig. 16. 



Orthonotacythere hannai (Israelsky) Alexand- 

 er, 1933, Jour. Paleontology, vol. 7, p. 200, 

 pi. 25, fig. la-c; pi. 26, fig. 6a-b; pi. 27, fig. 

 14a-b. 



Carapace in side view, ovate; dorsal margin 

 straight; ventral margin convex, curving sharply 

 upward posterad to a short thin caudate process 

 at the poster o-dor sal junction; anterior margin 

 obliquely rounded, weakly rimmed medially with 

 about four blunt spines below; posterior margin 

 obliquely rounded with a thin caudate extension 

 dorsally; carapace compressed, widest ventrally. 



Surface of valve with an ill-defined antero- 

 median sulcus dividing valve into two subequal 

 parts ; three strongly developed tubercles lie above 

 and parallel to the ventral margin ; a fourth well 

 developed tubercle lies above the postero-dorsal 

 junction; a fifth tubercle, more weakly developed, 

 lies posterad of the anterior margin submedially ; 

 about four or five weakly developed tubercles are 

 indiscriminately present on the valve, the most 

 pronounced of these lying at or adjacent to the 

 antero-dorsal angle. Surface of the valves with 

 well developed reticulations giving a striated ap- 

 pearance to the ventrum. Internal features char- 

 acteristic of the genus. 



Dimensions: Fig. 3: length 0.61 mm; height 

 0.39 mm; biconvexity 0.37 mm. 



Occurrence: Peedee rare to common, Black 

 Creek rare. Alexander lists this species both from 



Taylor and Navarro units in Texas. Localities 

 A, B, C, D, E, F, I, and K. U.S.N.M. 129032, 

 129033, 129034. 



Remarks : This distinctive species is very close 

 to O. cristata Alexander from the Paleocene. A 

 comparison with the Paleocene specimens of O. 

 cristata has shown the present specimens to be 

 slightly smaller, more ovate in side view and weak- 

 ly denticulate along the anterior margin; other- 

 wise they are seemingly identical. Alexander 

 (1934, p. 65) suggested that the stronger elevation 

 of the tubercles on O. cristata was a distinguish- 

 ing feature. In the present study the strength 

 and tuberculate pattern of the Cretaceous speci- 

 mens seem to be identical with specimens of O. 

 cristata from the Paleocene of Arkansas and 

 Tennessee. 



Orthonotacythere tarensis Brown, n. sp. 

 Plate 6, Figures 1, 2 



Carapace small, oblong to quadrate in side view ; 

 dorsal margin straight; ventral margin convex; 

 anterior margin broadly and evenly rounded; 

 posterior margin less broadly rounded, weakly 

 caudate dorsally. Anterior margin broad and com- 

 pressed. 



A broad, shallow, weakly defined sulcus, slight- 

 ly anterad of median, extends from dorsal mar- 

 gin to midpoint. Surface discretely tuberculate. 

 Anterad of sulcus dorsally, a low rounded tuber- 

 cle; a second low tubercle above and adjacent to 

 postero-ventral junction; two weak tubercles 

 above and parallel to the dorsal margin posterad. 

 Median and ventral portions of valve with about 

 ten fine parallel striations, strongest parallel to 

 ventro-lateral border. Entire surface very finely 

 reticulate. Hingement that of the genus. 



Dimensions : Holotype : length 0.66 mm ; height 

 0.27 mm ; biconvexity 0.25 mm. Fig. 2 : length 

 0.71 mm; height 0.24 mm; biconvexity 0.23 mm. 



Occurrence: Black Creek only, localities G, H, 

 I, J, and K. U.S.N.M. 129035, 129036. 



Remarks: Female dimorphs, generally broader 

 in relation to length, seem to show stronger tuber- 

 cular development. This species bears some re- 

 semblance to Monoceratina trinodosa Alexander 

 from the Weno of Texas. 



24 



