THE LAMELLIBRANCHS OF THE SILURIAN ROCKS OF GIRVAN. 487 



Byssonychia radiata, Hall, 1847. (PI. I., figs. 19-22.) 

 Ambonychia radiata, Hall, 1847, Pal. N. York, pt. i. p. 292, pi. lxxx. figs. 4a-6. 



Specific Characters. — Shell obliquely abovate, obliquely gibbose, anterior end 

 obsolete. The anterior surface is adpressed and much hollowed near the anterior 

 border, which is rounded. The anterior border appears to be straight and almost 

 vertical. Inferior border broadly rounded. Posterior margin truncate, oblique, making 

 an obtuse angle with the hinge line, which is straight. Umbones raised, produced 

 forwards, incurved, prosogyrous, contiguous. Dorsal slope compressed, flattened, 

 subalate. 



Interior. — Normal ; between the byssal orifice and the umbonal cavity is a thick 

 boss of shell, indicated in casts by a deep hollow. 



Exterior. — The surface is ornamented by many close, rounded, radiating ribs, 

 finer and closer on the dorsal slope. These radiating ribs are crossed by from 

 3 to 4 distant deep concentric sulci, giving rise to a crenulation on each rib as it crosses 

 them, more frequent near the lower margin. 



Dimensions. — Fig. 22, PI. I., a bivalve example, measures : antero-posteriorly, 

 28 mm. ; dorso-ventrally, 38 mm. ; from side to side, 19 mm. 



Locality. — Scotland : Upper Bala, Star-fish bed ; Drummuck. 



Observations. — This species is represented in the Gray Collection by several 

 examples. Hall separated his species from the Pterinea carinata of Goldfuss 

 on account of the much larger number of radiating ribs. He says that for shells of 

 equal size Goldfuss' specimen has 24, while his has 35 to 40. Only one of Hall's 

 figured specimens shows concentric interruptions to the ribs. 



Many of the specimens show the large circular byssal opening below the umbones. 



Byssonychia angusta, sp. nov. (PI. L, figs. 23, 24.) 



Specific Characters. — Shell acutely triangular, narrow, transversely oblique, 

 moderately convex. The anterior edge acute, compressed, its border depressed 

 beneath the overhanging umbones, and pierced by semicircular foramina in each valve, 

 for the byssus. The inferior border bluntly rounded. The posterior rounded below, 

 obliquely truncate, and almost straight above, where it joins the short, straight hinge 

 line at an obtuse angle. The umbones are produced, compressed, pointed, twisted 

 forwards and terminal. 



Interior. — An oblique cardinal tooth immediately below the umbo, with flattened 

 hinge plate, narrow, elongate ; posterior end not seen. 



Exterior. — Surface ornamented with numerous radiating thick ribs, separated by 

 grooves of equal size. Earely concentric lines of growth towards the lower border. 



Dimensions. — Fig. 24, PI. I., a right valve, measures : antero-posteriorly, 20 mm. ; 

 dorso-ventrally, 36 mm. 



TRANS. ROY. SOC. EDIN., VOL. XLVII. PART. III. (NO. 18). 72 



