194 SILURIAN MOLLUSCA.*^ 



culites poststriata (Emmons), but they are distinguished by the 

 parallelism of the dorsal and ventral margins, smaller beaks, 

 greater transverse diameters, &c. It also closely resembles the 

 Cypricardites sectifrons of Conrad, but that is figured with radia- 

 ting lines on the sides as well as the posterior slope ; the anterior 

 part of our specimen is unfortunately imperfect. 



Alt yr Anker, Meifod, Montgomeryshire. 



{Col, University of Cambridge.) 



Ambonychia'i acuticostata (M'Coy). 



Sp. Char. Ovate, moderately and evenly convex, most so towards 

 the beak ; surface radiated with numerous angular ridges, only 

 separated by the angular sulcus formed by the meeting of the 

 steep sides of the ridges (about six ridges in one-fourth of an 

 inch of the margin at half an inch from the beak). 



In form and number of the ridges this resembles the small 

 specimen of A. carinata (Gold, sp.) figured in Hallos 'Palseon- 

 tology ' (pi. 80. f. 5), but it is distinguished by its ribs being an- 

 gular and close together — they being rounded and separated by 

 flat interspaces in the American form. Only one imperfect spe- 

 cimen, measuring 7 lines from beak to ventral margin. 



In the green schists of Dinas Bran, Llangollen. 



(Col. University of Cambridge.) 



Aviculat Danhyi (M'Coy). 



Sp. Char. Obliquely ovate, anterior end broadly rounded, pos- 

 terior end more or less narrowed, rounded, ventral margin 

 evenly convex ; hinge-line rather less than half the width of 

 the shell, posterior wing scarcely twice the length of the an- 

 terior, both wings nearly rectangular with slightly concave 

 margins ; left valve gently convex, most gibbous near the 

 beaks, marked with minute concentric, irregular, interrupted 

 striae and wrinkles, crossed by a variable number of obtuse 

 ridges radiating from the beak, and generally becoming obso- 

 lete towards the margin ; right valve flat, with slight irregular 

 concentric wrinkles and strise of growth, without radiating 

 ridges. 



This species varies much in the amount of its obliquity and 

 transverse elongation, and the number of radiations on the 

 left valve ; these latter resemble the radiation of Pholadomya, for 

 which that valve might be taken when the ears are concealed. 

 Traces of a subcentral muscular impression occasionally visible. 

 Some of the varieties are so slightly oblique as to assume a ro- 

 tundato-quadrate form. Average length 1 inch 5 lines, width 

 2 inches 3 lines, length of hinge-line 1 inch. 



