28 JURASSIC BIVALVIA AND GASTROPODA 



near the umbones. Surface apparently bearing concentric threads and rugae (the 

 former, however, almost obliterated by erosion in the available specimens). 



Holotype and paratypes. Nos. LL.35001 and LL.35002-04 respectively, four 

 specimens in all. 



Locality and horizon. Didimtu hill, 2 miles S. of Bur Mayo, N.E. Kenya ; 

 Upper Lias, Toarcian, Didimtu Beds. 



Remarks. This species differs from the European Upper Liassic species Nuculana 

 rostralis (Lamarck) (synonym, N. claviformis (J. de C. Sowerby) (1824a : 119, pi. 476, 

 fig. 2)) in its more compressed form, its almost horizontal and more extended antero- 

 dorsal margin, and its flatter anterior margin. It is more closely comparable to N. 

 (Ryderia) doris (d'Orbigny) ( = Nucula compianata Goldfuss 1837, pi. 125, figs, na-c, 

 non Phillips) and N. (R.) graphica (Tate) (1870 : 407, pi. 26, fig. 12), both of Liassic 

 (Pliensbachian) age, but it differs from these species in its more quadrate anterior 

 end and its flatter ventral margin. 



Lemoine (1906 : 112) recorded N . doris from a locality south of Kola, Madagascar, 

 where it was associated with a Posidonia identified as P. alpina Gras, and where he 

 thought the beds might be Aalenian in age. As the specimens from that locality 

 have not been figured, it is impossible to say if they belong to the species now 

 described. 



Subgenus PRAESACCELLA Cox 1940 



Nuculana (Praesaccella) camelorum sp. nov. 

 PI. 2, figs, iofl, b 

 i960. Nuculana (Praesaccella) cf. juriana Cox ; Thompson & Dodson : 23 (listed). 



Diagnosis. Of medium size for the genus (length of largest specimen 9-3 mm.), 

 pyriform, with height about one-half of length ; inflation rather weak ; with 

 obtusely subangular umbones placed at about anterior third of length and an acutely 

 pointed posterior extremity which is almost at mid-height. Postero-dorsal outline 

 straight or slightly concave ; escutcheon narrow, not well seen in available specimens. 

 Antero-dorsal outline and anterior and ventral margins forming an uninterrupted 

 curve ; ventral margin in some specimens convex as far as its extremity, in others 

 with a small sinus at its posterior end. Ornament of very fine, regular concentric 

 threads. 



Holotype and paratypes. Numerous specimens exposed on a bedding-plane 

 of hard brownish limestone. The holotype (no. L. 98280) is the one represented in 

 the bottom right-hand corner of PI. 2. fig. 106. 



Locality and horizon. Camel track about 5 miles S. of Singu and 9 miles E. of 

 Tarbaj, N.E. Kenya ; Toarcian or Bajocian, top of Didimtu Beds. 



Remarks. Nuculana (Praesaccella) juriana Cox (1940 : 33, pi. 2, figs. 6-9), from 

 the Oxfordian of Cutch, India, is less inequilateral, higher in proportion to its length, 

 and ornamented with slightly coarser concentric threads. No more closely com- 

 parable species can be cited. 



