218 CAMBRO-SILURIAN FOSSILS. 



schists over the Bala limestone at Aber Hirnant E. of Bala, 

 North Wales ; and in the similar limestone of Cwm yr Aethen ; 

 oolitic limestone of Maes y fallen, Bala, Merionethshire : rare 

 in the limestone of Cerrig y Druidion, Denbighshire, 

 (CoL University of Cambridge.) 



Or this retrorsistria (M'Coy). 



Sp. Char. Rotundato-quadrate, depressed, no mesial ridge or 

 furrow in either valve ; hinge-line nearly or quite as wide as 

 the shell ; cardinal angles slightly obtuse ; cardinal area flat, 

 triangular, six or seven times wider than high in the receiving 

 valve, and inclined backwards at about 120°, only one-third 

 this height in the entering valve ; rostral tooth of entering 

 valve very large, triangular foramen of receiving valve with 

 an internal, semiconical, hood-shaped extension of the dental 

 lamellae within it ; receiving valve gently convex, greatest depth 

 about the middle of the length ; entering valve flat round the 

 margins, gently concave in the middle ; both valves with a few 

 concentric wrinkles of growth, about a line apart, and radiated 

 with slightly irregular obtuse strise, which branch into two or 

 three, at two or three intervals between the beak and margin, 

 so that each of the strong primary ones form from seven to 

 ten at the margin, separated by a rather deeper sulcus from 

 the adjoining ones, so as to produce a flat, indistinctly marked, 

 fasciculation ; intervening sulci about the same size as the 

 strise (obscurely punctured in some specimens), which are 

 straight in the middle, gradually assuming a divaricating curve 

 on the sides, which is so great near the angles, that a large 

 number of the lateral strise curve backwards from the beaks to 

 terminate along the distal half of each side of the hinge-line 

 instead of the lateral margin, all the strise crossed by indistinct 

 transverse lines of growth ; the size of the strise does not vary 

 much in the various parts of the shell, from fourteen to seven- 

 teen may be counted in two lines at four lines from the beak ; 

 cast of entering valve nearly smooth except at the margin, 

 which, in the middle, is marked with close slightly dichoto- 

 mous lines ; the narrow triangular boss of the foramen is cleft 

 by a deep, narrow, elliptical pit of the rostral tooth, flanked 

 by the pits of two short cardinal teeth, diverging at 70°, be- 

 tween which and the hinge-line are the smaller but more di- 

 verging bosses left by the cardinal pits ; a deep straight sulcus 

 extends from the rostral tooth nearly to the anterior margin, 

 marking a very long obtuse mesial ridge, on each side of the 

 rostral part of which is a subquadrate pair of muscular impres- 

 sions reaching less than half the length of the shell, the length 

 and width of the pair being nearly equal ; the lateral boundaries 



