THE LAMELLIBRANCHS OF THE SILURIAN ROCKS OP GIRVAN. 519 



genus with their main characters agreeing closely with Carboniferous forms in much 

 older rocks, and indicates that the ancestry of the genus must be looked for in still 

 older rocks. The ornament seems to have a permanence of character and to be pre- 

 served even in recent forms. 



I have referred several species to the genus, and it is of interest to note that the 

 tendency to the varietal development of the species is analogous to that which obtains 

 in the Carboniferous forms. 



Nuculana lobata, sp. nov. (PL IV., figs. 1-3, 3a.) 



Specific Characters. — Shell small, irregular in shape. The main portion of the shell 

 is triangular and gibbose ; the anterior superior angle is much compressed and expanded 

 into a lobe which projects forwards. The anterior margin is thin and descends 

 obliquely downwards and backwards. The inferior margin convex. The posterior end 

 is apparently narrowed, compressed, its margin obliquely truncate, straight above and 

 rounded below. The hinge line is erect and compressed anterior to the umbones, erect 

 and sloping posteriorly, moderately short, making an obtuse angle with the posterior 

 margin. The umbones are comparatively large, elevated, incurved, pointed, contiguous, 

 placed in front of the centre of the shell. The anterior umbonal slope is marked off 

 from the anterior lobe by a sudden and steep continuation. There is a very large, deep 

 escutcheon, bounded by a curved, rounded ridge which passes from the umbo to the 

 posterior margin. 



Interior. — The hinge is normal. Adductor muscle scars small and shallow. Pallial 

 line entire. 



Exterior. — The surface is covered with fine, close, regular lines of growth, which 

 may become interrupted by one or two deep concentric grooves, near the lower margin 

 of the valve. 



Dimensions. — Fig. 1, PL IV., measures : antero-posteriorly, 13 mm. ; dorso-ventrally, 

 10 mm. ; from side to side, 7 mm. 



Locality. — Upper Bala beds, Drummuck, Thraive Clem 



Observations. — This species is easily identified by the peculiar shape, caused by the 

 anterior lobe and the deep escutcheon. Fortunately, the collection contains a very good 

 specimen as a cast of the interior, so that the internal anatomy of the shell is demon- 

 strated. The presence of the well-developed escutcheon and the ornament have 

 determined me to place the shell in the genus Nuculana. 



Nuculana imbricata, Portlock, sp., 1846. (PL IV., figs. 4-7, 7a.) 



Nucula acuta, var. imbricata, Portlock, 1843, Geol. Rep. Londonderry, p. 430, pi. xxxiv. fig. 10. 

 „ suhacuta, M'Coy, 1846, Sil. Foss. Ireland, p. 19, pi. ii. fig. 3. 



Specific Characters. — Shell small, transversely ovate, acute. Anterior end rounded ; 

 posterior, narrowed and pointed. The inferior margin is convex. The hinge line 

 arched, the posterior limb descending rapidly, with a curve, whose convexity is upwards. 



TRANS. ROY. SOC. EDIN., VOL. XLVII. PART III. (NO. 18). 76 



