SOLENOMYA EXCISA. 441 



Solenomya primseva can be readily distinguisbed from the other fossil species 

 of the genus even in casts by the presence of broad flattened ribs, radiating 

 forwards from the umbo. 8. costellata is quite smooth in casts, although it has 

 somewhat the same shape as S. primseva. S. excisa differs so markedly from 

 both the other species in the possession of a well-marked narrowing of the 

 posterior extremity, that it is unnecessary to say more on the subject of 

 differential diagnosis. 



Solenomya kxcisa, de Konmcl, 1S85. Plate L, figs. 11, 12, 14 — 16. 



SoLEMYA AECUATA, de BycTchoU, 1S5;3. Melanges paleoiitol., pt. 2, p. 52. 



— Stmesii, Baihj, 18S0. Mem. Geol. Surv. Ireland, Expl. Sheets 42 and 



54, p. 22, two figures ouh'. 



— EXCTSA, de Koninch, 1885. Ann. Mus. IJoy. Hist. Nat. Belg., vol. si, 



p. 122, pi. xxiii, figs. 30, 39, 40, 42, 43 ; 

 and pi. ix, figs. 13, 14. 



Specific Cliaracfevs. — Shell inequilateral, obovate, lanceolate; gibbose above, 

 compressed below. The anterior end is by far the larger, and is much deeper 

 from above downwards than any other part of the shell, and moderately tumid ; 

 its border is bluntly rounded, but with a tendency to angulation at the junction 

 of the upper and anterior margins. The superior margin is produced, and very 

 gently convex. The posterior border is narrowed by the descent of the upper 

 margin, truncate. The upper margin is elevated and arched in front, but 

 descends rapidly posterior to the umbones, the postero-superior angle being well 

 marked. The umbones are small, tumid, elongate, contiguous, not raised, and 

 placed at the junction of the middle and posterior thirds of the valve ; excavated 

 posteriorly by a broad hollow escutcheon, which is separated from the rest of the 

 valve by an obscure curved ridge. The posterior end of the valve is short and 

 compressed, beaked, much more swollen above than below. 



Interior. — The anterior adductor muscle-scar is shallow and pear-shaped, 

 remote from the anterior edge ; the posterior scar is round and deep, bounded in 

 front by a low curved ridge. Pallial line entire. The inner surface of the shell 

 was marked by irregular concentric grooves and obsolete folds, crossed by 

 numerous very fine radiating strige, passing for the most part downwards and 

 forwards. 



Dimensions. — PI. L, fig. 11, measures — 



Antero-posteriorly . . . .50 mm. 



Dorso-ventrally . . . .22 mm. 



From side to side . . . .15 mm. 



57 



