Jt. B. Neidon — Fossils from Nubian Sandstone, Egypt. 391 



MUTELA MTCETOPOIDES, sp. nOV. (PI. XX, FigS. 11, 12.) 



Description. — Shell (right valve) shallow, soleniform, narrow, sub-cylindrical, 

 sub-arcuate, medially depressed ; hinge line straight, linear, parallel with ventral 

 border; umbo nearly central, small, compressed; anterior region obliquely inflated 

 from the umbo, flattened in front, outer margin rounded and gaping ; posterior part 

 obliquely and obtusely ridged, having a sloping depressed area in rear ; ventral 

 margin concave ; surface sculpture consisting of microscopically fine and close 

 striations following the contour lines of the shell, being often irregular and 

 sometimes of net«"ork character, and generally grouped within equidistant, rounded 

 plications of growth. 



Dimensions. — Height 25, length (approximate) 65, diameter (approximate) 10 mm. 



Remarks. — The foregoing description is based upon an external view 

 of a right valve attached to the rock, which, although of fragmentary 

 character, being fractured posteriorly, still preserves some important 

 features of the shell. Through compression and erosion the details of 

 the umbo are not definite, but its almost median position seems to be 

 clearly displayed. The opposing valve is in situ, as can be seen from 

 the presence of the antero-ventral margin, the remainder being entirely 

 hidden in matrix. These front terminal margins are important, since 

 they exhibit the well-marked gape, which is seen to extend ventrally, 

 as in the modern forms of this genus. On the central face of the 

 valve, and near the ventral border, two small vertical depressions occur, 

 produced probably by pressure or otherwise during the life of the 

 mollusc, giving rise to constrictions which locally disturb the regularity 

 of the lines of ornamentation. With regard to sculpture, the growth 

 plications may be said to be fairly well developed in places, whilst the 

 finer striations, only observable with a strong lens, are frequently 

 intertwined and irregular, forming more or less a structure of 

 filaments. 



The principal characters of this specimen include the straight hinge 

 line, the nearly median umbo, the posterior ridge, the inward curvature 

 or sinuation of the central region of the valve, the sub-cylindrical 

 contour, and anterior gape. In many of these details it appears to 

 more nearly resemble Mycetopus of Orbigny ^ than Scopoli's Mutela, 

 that is, judging from the type of the former genus {M. solenifonnis), 

 which lives in Central South American rivers; but remembering that 

 Mutela belongs essentially to African freshwaters, and is moreover 

 found in the alluvial deposits of the Xile,- it is more reasonable to 

 suppose that the fossil under consideration represents an ancestral form 

 of that genus rather than of another which belongs to a totally 

 different continent. In order to mark this resemblance the present 

 African fossil has been named Mutela mycetopoides. 



Occurrence. — The fossil is adherent to a mass of ironstone matrix 

 associated with a small form of Unio ( U. Crosthwaitei). Its surface 

 is mostly of a deep-black colour, besides being much cracked^ and 

 perforated in places in consequence of erosive agencies. Quite a 

 number of small, rounded, wart-like prominences are present on the 

 fossil, these representing pisolitic grains of iron-ore, the presence of 

 which would denote the highly ferruginous character of the waters 



' Voi/age dans V Amerique Meridionale, 1S4C, vol. v, pt. iii, Mollusques, pp. 600, 

 601, pl.'lxvi. 



2 Leith Adams, Quart. Joum. Geol. Soc, 1864, vol. xx, p. 15. 



