390 H. B. Newton — Fossils from Nubian Sandstone, Egypt. 



in front of umbones, margin rounded ; posterior region obtusely ridged from umbo 

 to postero- ventral angle, sloping and compressed behind ; ventral area sinuated at 

 margin ; antero-central face of valves obliquely and somewhat triangularly com- 

 pressed ; sculpture consisting of fine, equidistant, concentric, elevated striations, 

 sometimes wavy, bifurcating, and othenvise irregular, with obscure vertical ridging 

 between ; rounded, obscure plications of growth are also present. 

 Dimensions. — Height 25, length 47, diameter 18 mm. 



Remarks. — The specimen described has closed valves, between which 

 reposes a well-developed ligament tapering posteriorly and bearing 

 a few obscure transverse constrictions. The eroded umbones and 

 certain cavities present on both valves indicate the effects of past 

 river action before final deposition. The ornamentation is extremely 

 fine and well preserved, and without a lens appears fairly regular, but 

 a closer inspection shows that filament lines are given off from the 

 main concentric striations producing an appearance of bifurcation, 

 these lines being often gently wavy. Comparing it with Unio Hximei, 

 the species has relatively more convex valves, a less prominent central 

 depression, and a more cylindroid contour. This new form perhaps 

 compares more readily with Unio suhsimiatus of Koch & Dunker ^ 

 from German Wealden deposits in possessing the ventral sinuation, 

 although the valves are of greater depth, more convex, less rounded 

 and full anteriorly, whilst the posterior extremity is more produced. 

 Another example of the species, of somewhat shorter axis, possesses 

 a well-preserved test with distinct and regular sculpture lines bearing 

 intermittent vertical ridging; its umbones are a good deal eroded. 

 What appears to be another fonn of this species is associated in the 

 matrix with Mxdela mycetopoides, together with fragmentary remains 

 of the same shell standing out in relief on the weathered surface of 

 the rock. This specimen exhibits a left lateral view of evidently 

 a young form having a height of 13 and a length of 25 mm. The 

 sculpture lines are extremely fine and numerous, the postero-umboual 

 ridge is prominent, and a slightly oblique, narrow furrow proceeds 

 from the umbonal region to the ventral border, which may be of 

 accidental occurrence and not structural. The specimen also shows 

 that the umbo is eroded, and that there is little or no sinuation of the 

 ventral margin. 



Occurrence. — The specimens representing this species are in a good 

 state of preservation, althougli, like the other freshwater shells of this 

 collection, no internal characters are displayed. The matrix is of the 

 same reddish-brown ironstone rock, exhibiting a minutely pisolitic 

 structure, and the shells present a blackish metallic colour with 

 a lighter weathering. On the postero-dorsal surface of the left valve 

 of the principal specimen is an example of the Galeolaria situated in 

 close proximity to the elevated ligament. 



Genus MUTELA, Scopoli. 



Introductio ad Historiam Naturalem sistens genera Lapidum, Plantarum, et 



Aniraalium, etc., 1777, p. 397 (No. 83). 



Type = "LeMutel", Adanson : Hist. Nat. Senegal (Coquillages), 1757, p. 234, 



pi. xvii, fig. 21. (= Iridina exotica, Lamarck.) 

 Synonym — Iridina, Lamarck: Hist. Nat. Animaux sans Vertebres, 1819, vol. vi, 

 pt. i, pp. 88, 89. Tj-pe = Iridina exotica, Lamarck. 



^ Beitrdge norddeiitschen Oolithgebildes Versteinertmgen, 1837, p. 58, pi. vii, fig. 2. 



