AND MOLLUSCA FROM ANGOLA, PORTUGUESE WEST AFRICA. 567 



Ohoffat from the Albian of Dombe Grande, Angola. Both these species, however, 

 have more than one secondary rib between the primaries, the first-mentioned having 

 three and the other two. The nearly unicostate character of the present shell is an 

 interesting feature which, so far as can be ascertained, has not previously been 

 observed in this genus. 



Occurrence. — In a creamy-coloured limestone with ironstone nodules. 



Locality. — No. 20 — 80 metres above the Catumbella Dam. 



Neithea tricostata, Coquand. (PI. I, figs. 5-7.) 



Janira tricostata, Coquand : Gvologie et paleontologie de la region sud de la province de Constantine, 



1862, pi. xiii, figs. 3-4, p, 219 [non Pecten tricostatus, Bayle]. 

 Pecten coquandi, Peron : Desc. Moll. Foss. Cretach Tunisie, Explor. Scient. Tunisie, 1891, part 2, p. 224. 

 Pecten (Neithea) shawi, Pervinquiere : Etudes Pal. Tunisienne : Gastropodes et Lamellibranches des 'Terrains 



Oretaces, 1912, pi. ix, figs. 1-3, p. 136. 



Remarks. — There are two examples of this species in the collection, the better 

 one being of considerable size and in a fine state of preservation. It consists of a 

 lower valve with the central part of the shell substance removed by fracture. 

 Otherwise the characteristic external sculpture is entire and well displayed. The 

 interspaces dividing the six great primary costse are furnished with three equidistant 

 ribs of unequal size, the central being the most robust, whereas the rib on each 

 side is narrower and less elevated. A few nearly obsolete radial ribs may also be 

 traced on the large lateral areas, the surfaces of which, in common with the 

 remaining parts of the valve, are covered with fine concentric and closely arranged 

 striatums. The valve has a greater height than length, and in addition it may be 

 stated that the ventral margin is of strong polygonal contour, while the aliform 

 expansions, although only partially seen, give the idea of extreme smallness of size. 



Dimensions. — 



Length ...... 80 mm. 



Height . .... 99 „ 



Diameter . . . . . 40 ,, 



This species has been often confused with J. Sowerby's Pecten quadricostata, 

 from which it differs in its smaller auricles and in possessing a greater relative 

 height, and therefore not presenting a sub-quadratic contour ; while further the 

 intermittent costse are of unequal size instead of being generally of equal strength 

 as in Sowerby's shell. It is not possible to agree with Pervinquiere in regarding 

 Shaw's # old figure of a North African fossil as representing this species ; it is 

 much more likely to be a form related to, if not identical with, N. quadricostata, 

 on account of its much more nearly square outline, and from the fact that a similar 

 measurement characterises the height and length of the valve. This species offers con- 



* " Pectinites," Thomas Shaw : Voyage en Barbarie, 1743, vcl. ii, fig. 24, p. 128 (fide Pervinquiere) ; German 

 edition, 1765, pi. xxviii, fig. 24, p. 407. 



