192 SILURIAN MOLLUSCA*: 



wards and backwards towards the middle of the ventral mar- 

 gins ; a few of them meet at an acute angle^ about the middle 

 of the shell, with a few, more nearly vertical ridges proceeding 

 from the great diagonal ridge ; most of both sets of ridges go 

 towards the ventral margin ; they are separated by flat spaces 

 wider than their own diameter ; the posterior slope is divided 

 into three broad, rounded radiations by three shallow im- 

 pressed lines, crossed by irregular wrinkles parallel with the 

 posterior margin, all the ridges are slightly undulated by the 

 faint plicse of growth ; posterior dorsal lunette very narrow, 

 concave, horizontal (or perpendicular to the plane of the valves). 

 Length 3 lines, width 1 inch 4 lines. 



A specimen of the right valve shows rather more of the an- 

 gular ridges, though a smaller individual than one of the left. 

 Rare in the tilestone of Benson Knot, Kendal, Westmoreland. 

 (Col. University of Cambridge.) 



Leptodomus glohulosus (M'Coy). 



>^. Char. Globose, subtrigonal, width three-fourths of the length; 

 beaks very large, a little nearer the anterior than the posterior 

 end ; sides evenly tumid, most so in the middle ; posterior 

 slope undefined, but very steeply sloped ; anterior and poste- 

 rior ends subequal, slightly contracted, rounded, ventral mar- 

 gin convex ; hinge-line a little shorter than the shell, not ele- 

 vated, inflected portion narrow ; surface with a few concentric 

 lines of growth. Length 7J lines, width 6 lines, depth of one 

 valve 2 lines. 



This departs so widely from either the shortest or most gib- 

 bous varieties of the L. amygdalina (Sow. sp.), that it seems desi- 

 rable to give it a distinctive name ; there is no other closely allied 

 form. The general appearance approaches that of the Nucula 

 ovalis of the same group, but in the latter the diagonal posterior 

 ridge is more angular, and I have ascertained that it really pos- 

 sesses teeth as in Nucula. 



Hard green micaceous Upper Ludlow rocks of Tenterfell, 

 Kirkby Moor, and Benson Knot, Kendal, Westmoreland. 



{Col. University of Cambridge.) 



Leptodomus truncatus (M^Coy). 



Sp. Char. Oblong or subtrigonal, compressed ; beaks very large, 

 gibbous, prominent, terminal, the anterior end being almost 

 vertically subtruncate under it, width of the anterior end 

 (where it is greatest) nearly two-thirds the length of the 

 shell ; posterior end obliquely subtruncate or rounded ; ventral 

 margin gently convex, with a scarcely perceptible sinus a little 



