THE LAMELLIBRANCHS OF THE SILURIAN ROCKS OF GIRVAN. 491 



Observations. — This species is represented in the collection by numerous examples 

 from Penkhill and Mulloch Hill, but only two specimens were found at Woodland 

 Point. Although there are two casts of the interior, I have not been able to make out 

 the muscle scars, and only a part of the hinge plate is exposed. 



Several specimens show the radiating lines on the anterior part of the shell. 



The largest specimen found measures 47 mm. antero-posteriorly ; in full-grown 

 shells the radiating ornament is much better marked than in those belonging to 

 immature examples. 



Ambonychia cuneata, sp. nov. (PI. II., figs. 4-6, 6a, 6b.) 



Specific Characters. — Shell of moderate size, obliquely subquadrate, gibbose 

 anteriorly, compressed posteriorly, very inequilateral. The anterior end is adpressed, 

 with a broad anterior surface, hollowed above on each side of the narrow projecting 

 border. The inferior border is blunt and slightly convex ; the posterior oblique, almost 

 straight. The hinge line is short and straight. The umbones are gibbose, pointed, 

 twisted forwards and raised. The dorsal slope is much compressed and hollowed. 



Interior. — Not seen. 



Exterior. — The surface appears to have been almost smooth, with obscure con- 

 centric strise of growth. 



Dimensions. — Fig. 5, PI. II., measures: antero-posteriorly, 18 mm.; dorso- 

 ventrally, 17 mm. ; from side to side, 12 mm. 



Localities. — ? Star-fish bed, Upper Bala ; Mulloch Hill, Lower Llandovery; Woodland 

 Point, Middle Llandovery ; Penkhill, Upper Llandovery. 



Observations. — Ambonychia acutirostris, Hall, from the Niagara beds of North 

 America, has a certain resemblance to this species, but it is too oblique, and its dorso- 

 ventral measurements are comparatively much longer. I have not been able to see 

 any trace of the striated hinge plate of Ambonychia, and have referred the shell to 

 that genus provisionally. 



Ambonychia simplex, sp. nov. (PI. II., figs. 7, 7a-c, 8, 9.) 



Specific Characters. — Shell small, narrow oblique, ovate acute, moderately gibbose 

 in front, compressed posteriorly, very inequilateral. The anterior end adpressed so 

 that the line of junction of the valves is posterior to the anterior border, which 

 descends obliquely downwards and backwards, and is slightly sinuate. Inferior border 

 bluntly rounded ; posterior border obliquely truncate, gently convex. Hinge line short 

 and straight ; postero-superior angle well marked, slightly obtuse. The umbones are 

 large, prominent, pointed, prosogyrous, and terminal. 



Interior. — Normal. 



Exterior. — The surface seems to have been smooth. 



Dimensions. — Fig. 7, PI. IJ., a right valve, measures: antero-posteriorly, 13 mm. ; 

 dorso-ventrally, 17 mm. 



