498 DR WHEELTON HIND ON 



? Leptodesma modiolaris, sp. nov. (PL I., figs. 13-15, 15a.) 



Specific Characters. — Shell of medium size, roughly trigonal, modioliform. Anterior 

 end nasute, small, separated from the rest of the valve by a deep, broad byssal sinus, 

 which notches deeply the anterior part of the lower border ; posteriorly the lower border 

 is broadly convex. Posterior border truncate, almost straight. Hinge line long, straight. 

 Postero-superior angle almost a right angle. Umbones small, directed forwards, but 

 not terminal. Posterior to the byssal sinus, the valve is obliquely swollen, the 

 swelling becoming broader and flatter as it approaches the margin. Postero-superior 

 angle compressed. 



Interior. — Unknown. 



Exterior. — The surface is almost smooth, but here and there concentric wrinkles and 

 lines of growth are seen. These are much more pronounced in the neighbourhood of 

 the byssal sinus. 



Dimensions. — Fig. 13, PL I., measures : antero-posteriorly, 22 mm.; dorso-ventrally, 

 15 mm.; gibbosity of valve, 3 mm. 



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



Observatiofis. — This species is founded on the cast of a right valve, from which I 

 have taken a wax impression. It is a well-marked form, but whether I am right in 

 referring it to Leptodesma rather than to Leiopteria I cannot say without further 

 material which would enable me to ascertain the character of the hinge plate. 



Leptodesma transversa, sp. nov. (PL I., figs. 17, 18.) 



Specific Characters. — Shell below medium size, transversely ovate. Anterior end 

 narrow and short, its margin rounded, the posterior expanded and subalate, obliquely 

 gibbose. The anterior margin elliptical, the inferior gently convex, the posterior 

 rounded below, falcate above. The hinge line straight, produced posteriorly in a long 

 thin process. 



The umbones are small, swollen, scarcely raised above the hinge line, placed far 

 forwards. 



Interior. — Not exposed. 



Exterior. — The surface is almost smooth, but the microscope reveals fine concentric 

 lines of growth. 



Dimensions. — Fig. 18, PL I., measures : antero-posteriorly, 12 mm. ; dorso-ventrally, 

 8 mm. 



Locality. — Lower Llandovery of Mulloch Hill. 



Observations. — Only a single example, a right valve, has been obtained. It is more 

 transverse and less triangular than L. modiolaris, and has a very elongate process at the 

 posterior end of the hinge line of which the latter has no trace. 



