THE LAMELLIBRANCHS OF THE SILURIAN ROCKS OF GIRVAN. 489 



a slightly obtuse angle with the hinge line. The latter long and straight. Umbones 

 pointed terminal and excavated internally. The anterior part of the valve is adpressed 

 and folded inwards, and presents a broad anterior surface. 



Interior. — The posterior part of the hinge plate has a fine elongate lateral 

 tooth. 



Exterior. — The surface is ornamented with fine concentric lines of growth, crossed 

 by almost obsolete radiating ribs, best marked in front. 



Dimensions. — Fig. 27, PL I., a right valve, measures : antero-posteriorly, 27 mm. ; 

 dorso-ventrally, 23 mm. ; gibbosity of valve, 7 mm. 



Locality. — Scotland : Llandeilo beds, Craighead ; Lower Llandovery, Mulloch Hill ; 

 Middle Llandovery, Woodland Point. 



Observations. — This is an easily recognised form which reaches a size about three 

 times as large as the figured specimen. The species was originally described by Salter 

 from Bird's Hill. If the shell is described with the hinge line horizontal, it is seen that 

 Salter's remarks that the " beak of the present species falls considerably within the 

 anterior margin," and " the great gibbosity, too, is beneath the beak, not in a diagonal 

 line from this to the siphonal end," are not correct, but depend entirely upon the 

 orientation of the specimen. Nearly all the shells that I have diagnosed as belonging 

 to this species show more or less clearly defined radial ribbing, even in the young. 

 The Llandeilo specimens are very large, Lower Llandovery small, and Middle 

 Llandovery nearly equal to the largest specimen. P. triton is much flatter than 

 P. quadratics. 



Ambonychia amygdalina, Hall, 1847. (PL I., figs. 28, 29.) 

 Ambonychia amygdalina, Hall, 1847, Pal. New York, vol. i. p. 165, pi. xxxvi. fig. 6a, b, c. 



Specific Characters. — Shell narrow, obliquely ovate, and gibbose. Anterior end 

 obsolete, the margin adpressed to form a broad surface. The inferior margin elliptical, 

 the posterior elongate, oblique, nearly straight. The hinge line short, straight, making 

 an obtuse angle with the posterior border. Umbones acute, pointed, curved forwards, 

 contiguous, terminal. Near the upper angle the umbonal gibbosity is almost angular 

 on the anterior slope. Dorsal slope rapidly compressed. 



Interior. — No details have been obtained. 



Exterior. — Apparently covered by fine concentric lines of growth. 



Dimensions. — Fig. 24, PL L, a right valve, measures : antero-posteriorly, 41 mm. ; 

 dorso-ventrally, 23 mm. 



Locality. — Scotland : Llandeilo beds, Balclatchie, Craighead, Ardmillan, Dowhill, 

 Minuntion 



Observations. — This species is evidently not uncommon, but the specimens are very 

 poor, in the form of casts, which unfortunately give no details of the hinge plate or 

 muscle scars. 



