108 JURASSIC BIVALVIA AND GASTROPODA 



Superfamily ARCTICACEA 



Family ARCTICIDAE Newton 1891 



Genus PRONOELLA Fischer 1887 



Pronoella pindiroensis sp. nov. 

 PI. 17, figs. 12, 13, 14a, b, 15, 16, 17a, b 



Diagnosis. Of medium size (length of largest specimen 33 mm.), elongate-cunei- 

 form, strongly inequilateral, height from one-half to two-thirds of length, beaks 

 between anterior quarter and fifth of length ; some but not all specimens posteriorly 

 rostrate, some strongly inflated, others only moderately so even when apparently 

 uncrushed. Umbo not prominent, broadly rounded to obtusely angular, slightly 

 incurved to prosogyrous beak. Postero-dorsal outline parasigmoidal to feebly and 

 evenly convex and ventral margin posteriorly sinuate or feebly and evenly convex 

 according to presence or absence of posterior rostrum ; posterior margin low, almost 

 straight, oblique ; antero-dorsal outline slightly concave ; anterior margin broadly 

 convex. Lunule moderately broad and deep, not distinctly bordered ; bordering 

 ridges of escutcheon absent or ill-defined. An obtuse angulation, straight or para- 

 sigmoidal according to presence or absence of posterior rostrum, and scarcely defined 

 in some specimens, runs from beak to postero-ventral corner. Surface ornamented 

 with irregular concentric threads or rugae which are present on all the anterior part 

 but tend to disappear on the posterior part of the flank, and are absent from the 

 posterior area. 



Holotype and paratypes. Holotype, no. LL.35135. There are numerous 

 paratypes including nos. LL. 35136-40, but many are crushed. All ex B.P. Coll. 



Localities and horizon. Near site of Mandawa well no. 1, Tanganyika (type- 

 locality) ; Lihimaliao creek, at a point near Mbaru creek, Mandawa area, Tangan- 

 yika ; Bajocian (?), Pindiro Shales. 



Description. The description given in the diagnosis may be supplemented by 

 an account of the dentition, which is that of a typical Pronoella. Right valve with 

 an elongate posterior cardinal (36), obscurely bifid and almost parallel with postero- 

 dorsal margin ; a triangular median cardinal (1) with its broad apex well separated 

 from the beak and tapering anteriorly to be continued by a thin anterior lateral 

 (Ai) ; a very thin, elongate anterior cardinal (3a) passing into an equally thin lateral 

 (Aiii), the two separated by a very narrow recess from the adjacent margin ; and 

 an elongate posterior lateral (Pi), separated by a recess from the margin : left valve 

 with three cardinal teeth of which the posterior (46) is thin and elongate, lying along 

 the lower margin of the ligamental nymph, the median (26) is stout, bifid, and in- 

 clined backward from the beak, and the anterior [2d) is moderately stout and elon- 

 gate and lies close to the antero-dorsal margin. 



Remarks. At first sight the specimens included in this species appear to belong 

 to at least two different species, but there appears to be complete intergradation 

 between specimens with a well-developed posterior rostrum and those with no ros- 



