64 CI. F. MATTHEW: ILLUSTRATIONS OF 



These contrasted features of the palaeozoic Beyrichiutc of this Section, may be pre- 

 sented as follows : — 



" Bei/richiœjugosœ." {Beyriclna in part). Valves oblong, semicircular below, somewhat 

 flattened, crossed by three (or more) iinsymmetrical ridges variable in form and continuity. 

 ■ — Upper Cambrian' to Silurian, etc. 



Eeijrkliiiiœ ungiiiformes. {Hippotiiclmrion, u. gen.) Valves, in which the breadth nearly 

 equals the length ; broadly semi-elliptical toward the base, flattened, crossed (in the species 

 known) by three symmetrical ridges, of which the middle one is small and inconspicuous. 

 — Lower Cambrian. 



" Bei/riclikr corrugdta" {Beyrichia in part.) Valves oblong, broad end posterior, semi- 

 circular below, rounded on the surface, and marked by two transverse furrows. — Ordovi- 

 cian to Silurian etc. . 



Beyrichinœ nliformes. {Bei/ricliona, n. gen.) Valves with breadth and length nearly 

 equal, broad end anterior, subtri angular toward the base, rounded on the surface and 

 having two furrows, short and faintly impressed. — Lower Cambrian. 



HIPPONICHARION, n. gen. 



Valves wide, semi-elliptical, subequilateral ; outer area of the valve, except on the 

 side next the hinge, strongly elevated into prominent marginal ridges ; central area, 

 including the upper side of the valve, greatly depressed, and having an inconspicuoiTS 

 central ridge near the hinge line ; valves nearly equal. 



HIPPONICHARION EOS, n. sp. (Plate VL Figs. 19, 19a and b.) 



The following additional characters may be indicated as specific : — 



Valves regularly semi-elliptical, widest a little behind the median line, and having a 

 more rounded curve on the anterior than on the posterior side ; the hinge line is long, and 

 the extremities of the A'alve are rounded in towards it. The valves are nearly of equal 

 size, but the left slightly overlaps the right. 



About half of the valve, including the middle and upper side, is flattened and 

 depressed below the bounding ridges ; this flattened part is triangular, rises somewhat 

 toward the posterior border, and then suddenly descends as a .shallow depression to meet 

 the fold at the lower margin of the valve ; the flatness of this central depression is broken 

 near the hinge line by a low ridge, which is somewhat nearer the anterior than the pos- 

 terior end of the A'alve ; this ridge is crowned by an inconspicuous tubercle close to the 

 hinge line, and has a narrow, thread-like ridgelet along its crest, the ridge extends toward 

 the lower angle of the flattened space of the valve and fades owi half way across it. 



The anterior and posterior ridges are of about equal length ; they are high and have 

 an abrupt slope to the central flattened area of the valve ; and a longer abrupt slope to the 

 marginal furrow. 



The margin of the valve has a narrow raised rim, which is separated from the high 



Supposing tliat Beyrichia Angclini belongs to this section. 



